MINGW: Define variable only when targeting Windows platforms

The `MINGW` variable indicates that the compiler targets MinGW, a GNU
ABI on Windows.  Since commit aff3147917 (Modernize GNU compiler info on
Windows, 2009-12-02, v2.8.2~636), we load the `Platform/Windows-GNU`
module for compilers targetin MinGW, so set the variable there instead.
This is equivalent to `Platform/Windows-MSVC` setting the `MSVC`
variable.  Also remove `if(MINGW)` checks from the module, which have
not been necessary since the enclosed logic was moved to that module.

The undocumented `CMAKE_COMPILER_IS_MINGW` internal variable is now
unused, so remove it too.

Fixes: #22647

Modules/CMakeCCompiler.cmake.in

@@ -30,13 +30,9 @@ set(CMAKE_COMPILER_IS_GNUCC @CMAKE_COMPILER_IS_GNUCC@)
 set(CMAKE_C_COMPILER_LOADED 1)
 set(CMAKE_C_COMPILER_WORKS @CMAKE_C_COMPILER_WORKS@)
 set(CMAKE_C_ABI_COMPILED @CMAKE_C_ABI_COMPILED@)
-set(CMAKE_COMPILER_IS_MINGW @CMAKE_COMPILER_IS_MINGW@)
 
 set(CMAKE_C_COMPILER_ENV_VAR "CC")
 
-if(CMAKE_COMPILER_IS_MINGW)
-  set(MINGW 1)
-endif()
 set(CMAKE_C_COMPILER_ID_RUN 1)
 set(CMAKE_C_SOURCE_FILE_EXTENSIONS c;m)
 set(CMAKE_C_IGNORE_EXTENSIONS h;H;o;O;obj;OBJ;def;DEF;rc;RC)

Modules/CMakeCXXCompiler.cmake.in

@@ -31,13 +31,9 @@ set(CMAKE_COMPILER_IS_GNUCXX @CMAKE_COMPILER_IS_GNUCXX@)
 set(CMAKE_CXX_COMPILER_LOADED 1)
 set(CMAKE_CXX_COMPILER_WORKS @CMAKE_CXX_COMPILER_WORKS@)
 set(CMAKE_CXX_ABI_COMPILED @CMAKE_CXX_ABI_COMPILED@)
-set(CMAKE_COMPILER_IS_MINGW @CMAKE_COMPILER_IS_MINGW@)
 
 set(CMAKE_CXX_COMPILER_ENV_VAR "CXX")
 
-if(CMAKE_COMPILER_IS_MINGW)
-  set(MINGW 1)
-endif()
 set(CMAKE_CXX_COMPILER_ID_RUN 1)
 set(CMAKE_CXX_SOURCE_FILE_EXTENSIONS C;M;c++;cc;cpp;cxx;m;mm;mpp;CPP;ixx;cppm)
 set(CMAKE_CXX_IGNORE_EXTENSIONS inl;h;hpp;HPP;H;o;O;obj;OBJ;def;DEF;rc;RC)

Modules/CMakeDetermineCCompiler.cmake

@@ -128,9 +128,6 @@ if(NOT CMAKE_C_COMPILER_ID_RUN)
   if(CMAKE_C_COMPILER_ID STREQUAL "GNU")
     set(CMAKE_COMPILER_IS_GNUCC 1)
   endif()
-  if(CMAKE_C_PLATFORM_ID MATCHES "MinGW")
-    set(CMAKE_COMPILER_IS_MINGW 1)
-  endif()
 else()
   if(NOT DEFINED CMAKE_C_COMPILER_FRONTEND_VARIANT)
     # Some toolchain files set our internal CMAKE_C_COMPILER_ID_RUN

Modules/CMakeDetermineCXXCompiler.cmake

@@ -125,9 +125,6 @@ if(NOT CMAKE_CXX_COMPILER_ID_RUN)
   if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
     set(CMAKE_COMPILER_IS_GNUCXX 1)
   endif()
-  if(CMAKE_CXX_PLATFORM_ID MATCHES "MinGW")
-    set(CMAKE_COMPILER_IS_MINGW 1)
-  endif()
 else()
   if(NOT DEFINED CMAKE_CXX_COMPILER_FRONTEND_VARIANT)
     # Some toolchain files set our internal CMAKE_CXX_COMPILER_ID_RUN

Modules/CMakeDetermineFortranCompiler.cmake

@@ -235,9 +235,6 @@ if(NOT CMAKE_Fortran_COMPILER_ID_RUN)
   if(CMAKE_Fortran_COMPILER_ID MATCHES "GNU")
     set(CMAKE_COMPILER_IS_GNUG77 1)
   endif()
-  if(CMAKE_Fortran_PLATFORM_ID MATCHES "MinGW")
-    set(CMAKE_COMPILER_IS_MINGW 1)
-  endif()
 endif()
 
 if (NOT _CMAKE_TOOLCHAIN_LOCATION)

Modules/CMakeFortranCompiler.cmake.in

@@ -18,15 +18,11 @@ set(CMAKE_COMPILER_IS_GNUG77 @CMAKE_COMPILER_IS_GNUG77@)
 set(CMAKE_Fortran_COMPILER_LOADED 1)
 set(CMAKE_Fortran_COMPILER_WORKS @CMAKE_Fortran_COMPILER_WORKS@)
 set(CMAKE_Fortran_ABI_COMPILED @CMAKE_Fortran_ABI_COMPILED@)
-set(CMAKE_COMPILER_IS_MINGW @CMAKE_COMPILER_IS_MINGW@)
 
 set(CMAKE_Fortran_COMPILER_ENV_VAR "FC")
 
 set(CMAKE_Fortran_COMPILER_SUPPORTS_F90 @CMAKE_Fortran_COMPILER_SUPPORTS_F90@)
 
-if(CMAKE_COMPILER_IS_MINGW)
-  set(MINGW 1)
-endif()
 set(CMAKE_Fortran_COMPILER_ID_RUN 1)
 set(CMAKE_Fortran_SOURCE_FILE_EXTENSIONS f;F;fpp;FPP;f77;F77;f90;F90;for;For;FOR;f95;F95)
 set(CMAKE_Fortran_IGNORE_EXTENSIONS h;H;o;O;obj;OBJ;def;DEF;rc;RC)

Modules/Platform/Windows-GNU.cmake

@@ -8,6 +8,8 @@ if(__WINDOWS_GNU)
 endif()
 set(__WINDOWS_GNU 1)
 
+set(MINGW 1)
+
 set(CMAKE_IMPORT_LIBRARY_PREFIX "lib")
 set(CMAKE_SHARED_LIBRARY_PREFIX "lib")
 set(CMAKE_SHARED_MODULE_PREFIX  "lib")
@@ -19,16 +21,12 @@ set(CMAKE_SHARED_LIBRARY_SUFFIX ".dll")
 set(CMAKE_SHARED_MODULE_SUFFIX  ".dll")
 set(CMAKE_STATIC_LIBRARY_SUFFIX ".a")
 
-if(MSYS OR MINGW)
-  set(CMAKE_EXTRA_LINK_EXTENSIONS ".lib") # MinGW can also link to a MS .lib
-endif()
+set(CMAKE_EXTRA_LINK_EXTENSIONS ".lib") # MinGW can also link to a MS .lib
 
-if(MINGW)
-  set(CMAKE_FIND_LIBRARY_PREFIXES "lib" "")
-  set(CMAKE_FIND_LIBRARY_SUFFIXES ".dll.a" ".a" ".lib")
-  set(CMAKE_C_STANDARD_LIBRARIES_INIT "-lkernel32 -luser32 -lgdi32 -lwinspool -lshell32 -lole32 -loleaut32 -luuid -lcomdlg32 -ladvapi32")
-  set(CMAKE_CXX_STANDARD_LIBRARIES_INIT "${CMAKE_C_STANDARD_LIBRARIES_INIT}")
-endif()
+set(CMAKE_FIND_LIBRARY_PREFIXES "lib" "")
+set(CMAKE_FIND_LIBRARY_SUFFIXES ".dll.a" ".a" ".lib")
+set(CMAKE_C_STANDARD_LIBRARIES_INIT "-lkernel32 -luser32 -lgdi32 -lwinspool -lshell32 -lole32 -loleaut32 -luuid -lcomdlg32 -ladvapi32")
+set(CMAKE_CXX_STANDARD_LIBRARIES_INIT "${CMAKE_C_STANDARD_LIBRARIES_INIT}")
 
 set(CMAKE_DL_LIBS "")
 set(CMAKE_LIBRARY_PATH_FLAG "-L")
@@ -48,21 +46,19 @@ endif()
 
 macro(__windows_compiler_gnu lang)
 
-  if(MSYS OR MINGW)
-    # Create archiving rules to support large object file lists for static libraries.
-    set(CMAKE_${lang}_ARCHIVE_CREATE "<CMAKE_AR> qc <TARGET> <LINK_FLAGS> <OBJECTS>")
-    set(CMAKE_${lang}_ARCHIVE_APPEND "<CMAKE_AR> q <TARGET> <LINK_FLAGS> <OBJECTS>")
-    set(CMAKE_${lang}_ARCHIVE_FINISH "<CMAKE_RANLIB> <TARGET>")
-
-    # Initialize C link type selection flags.  These flags are used when
-    # building a shared library, shared module, or executable that links
-    # to other libraries to select whether to use the static or shared
-    # versions of the libraries.
-    foreach(type SHARED_LIBRARY SHARED_MODULE EXE)
-      set(CMAKE_${type}_LINK_STATIC_${lang}_FLAGS "-Wl,-Bstatic")
-      set(CMAKE_${type}_LINK_DYNAMIC_${lang}_FLAGS "-Wl,-Bdynamic")
-    endforeach()
-  endif()
+  # Create archiving rules to support large object file lists for static libraries.
+  set(CMAKE_${lang}_ARCHIVE_CREATE "<CMAKE_AR> qc <TARGET> <LINK_FLAGS> <OBJECTS>")
+  set(CMAKE_${lang}_ARCHIVE_APPEND "<CMAKE_AR> q <TARGET> <LINK_FLAGS> <OBJECTS>")
+  set(CMAKE_${lang}_ARCHIVE_FINISH "<CMAKE_RANLIB> <TARGET>")
+
+  # Initialize C link type selection flags.  These flags are used when
+  # building a shared library, shared module, or executable that links
+  # to other libraries to select whether to use the static or shared
+  # versions of the libraries.
+  foreach(type SHARED_LIBRARY SHARED_MODULE EXE)
+    set(CMAKE_${type}_LINK_STATIC_${lang}_FLAGS "-Wl,-Bstatic")
+    set(CMAKE_${type}_LINK_DYNAMIC_${lang}_FLAGS "-Wl,-Bdynamic")
+  endforeach()
 
   # No -fPIC on Windows
   set(CMAKE_${lang}_COMPILE_OPTIONS_PIC "")