Cygwin: Remove redundant definitions of CYGWIN and UNIX variables

These are set by modules loaded for `CMAKE_SYSTEM_NAME`.  We do not
need to set them again if the compiler defines `__CYGWIN__`.

Also remove the now-unused undocumented `CMAKE_COMPILER_IS_CYGWIN`
internal variable.

Modules/CMakeCCompiler.cmake.in

@@ -31,11 +31,6 @@ 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_COMPILER_IS_CYGWIN @CMAKE_COMPILER_IS_CYGWIN@)
-if(CMAKE_COMPILER_IS_CYGWIN)
-  set(CYGWIN 1)
-  set(UNIX 1)
-endif()
 
 set(CMAKE_C_COMPILER_ENV_VAR "CC")
 

Modules/CMakeCXXCompiler.cmake.in

@@ -32,11 +32,6 @@ 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_COMPILER_IS_CYGWIN @CMAKE_COMPILER_IS_CYGWIN@)
-if(CMAKE_COMPILER_IS_CYGWIN)
-  set(CYGWIN 1)
-  set(UNIX 1)
-endif()
 
 set(CMAKE_CXX_COMPILER_ENV_VAR "CXX")
 

Modules/CMakeDetermineCCompiler.cmake

@@ -130,8 +130,6 @@ if(NOT CMAKE_C_COMPILER_ID_RUN)
   endif()
   if(CMAKE_C_PLATFORM_ID MATCHES "MinGW")
     set(CMAKE_COMPILER_IS_MINGW 1)
-  elseif(CMAKE_C_PLATFORM_ID MATCHES "Cygwin")
-    set(CMAKE_COMPILER_IS_CYGWIN 1)
   endif()
 else()
   if(NOT DEFINED CMAKE_C_COMPILER_FRONTEND_VARIANT)

Modules/CMakeDetermineCXXCompiler.cmake

@@ -127,8 +127,6 @@ if(NOT CMAKE_CXX_COMPILER_ID_RUN)
   endif()
   if(CMAKE_CXX_PLATFORM_ID MATCHES "MinGW")
     set(CMAKE_COMPILER_IS_MINGW 1)
-  elseif(CMAKE_CXX_PLATFORM_ID MATCHES "Cygwin")
-    set(CMAKE_COMPILER_IS_CYGWIN 1)
   endif()
 else()
   if(NOT DEFINED CMAKE_CXX_COMPILER_FRONTEND_VARIANT)

Modules/CMakeDetermineFortranCompiler.cmake

@@ -237,8 +237,6 @@ if(NOT CMAKE_Fortran_COMPILER_ID_RUN)
   endif()
   if(CMAKE_Fortran_PLATFORM_ID MATCHES "MinGW")
     set(CMAKE_COMPILER_IS_MINGW 1)
-  elseif(CMAKE_Fortran_PLATFORM_ID MATCHES "Cygwin")
-    set(CMAKE_COMPILER_IS_CYGWIN 1)
   endif()
 endif()
 

Modules/CMakeFortranCompiler.cmake.in

@@ -19,11 +19,6 @@ 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_COMPILER_IS_CYGWIN @CMAKE_COMPILER_IS_CYGWIN@)
-if(CMAKE_COMPILER_IS_CYGWIN)
-  set(CYGWIN 1)
-  set(UNIX 1)
-endif()
 
 set(CMAKE_Fortran_COMPILER_ENV_VAR "FC")