CMakeForceCompiler: Deprecate this module and its macros

This module was added by commit v2.6.0~1489 (...supporting embedded
compilers..., 2007-07-02) to avoid needing to build the compiler id
detection source on toolchains that need special flags or linker files
to produce executables.  Since then CMake has learned other ways to
identify such compilers (e.g. passing vendor-specific version flags and
checking for vendor-specific banners in the output).  Furthermore, CMake
now detects a lot more than just the compiler id during language
initialization in order to support things like <LANG>_STANDARD and
COMPILE_FEATURES.  Therefore CMakeForceCompiler is no longer a viable
approach because it skips such detection.

One remaining challenge to support toolchains that require special
linker files is to build the test for a working compiler, compiler ABI
detection binary, and other try_compile cases.  This will have to be
worked out as such cases are encountered.

Help/release/dev/deprecate-CMakeForceCompiler.rst

@@ -0,0 +1,5 @@
+deprecate-CMakeForceCompiler
+----------------------------
+
+* The :module:`CMakeForceCompiler` module and its macros are now deprecated.
+  See module documentation for an explanation.

Modules/CMakeForceCompiler.cmake

@@ -2,11 +2,17 @@
 # CMakeForceCompiler
 # ------------------
 #
+# Deprecated.  Do not use.
 #
+# The macros provided by this module were once intended for use by
+# cross-compiling toolchain files when CMake was not able to automatically
+# detect the compiler identification.  Since the introduction of this module,
+# CMake's compiler identification capabilities have improved and can now be
+# taught to recognize any compiler.  Furthermore, the suite of information
+# CMake detects from a compiler is now too extensive to be provided by
+# toolchain files using these macros.
 #
-# This module defines macros intended for use by cross-compiling
-# toolchain files when CMake is not able to automatically detect the
-# compiler identification.
+# -------------------------------------------------------------------------
 #
 # Macro CMAKE_FORCE_C_COMPILER has the following signature:
 #
@@ -64,6 +70,8 @@
 #  License text for the above reference.)
 
 macro(CMAKE_FORCE_C_COMPILER compiler id)
+  message(DEPRECATION "The CMAKE_FORCE_C_COMPILER macro is deprecated.  "
+    "Instead just set CMAKE_C_COMPILER and allow CMake to identify the compiler.")
   set(CMAKE_C_COMPILER "${compiler}")
   set(CMAKE_C_COMPILER_ID_RUN TRUE)
   set(CMAKE_C_COMPILER_ID ${id})
@@ -76,6 +84,8 @@ macro(CMAKE_FORCE_C_COMPILER compiler id)
 endmacro()
 
 macro(CMAKE_FORCE_CXX_COMPILER compiler id)
+  message(DEPRECATION "The CMAKE_FORCE_CXX_COMPILER macro is deprecated.  "
+    "Instead just set CMAKE_CXX_COMPILER and allow CMake to identify the compiler.")
   set(CMAKE_CXX_COMPILER "${compiler}")
   set(CMAKE_CXX_COMPILER_ID_RUN TRUE)
   set(CMAKE_CXX_COMPILER_ID ${id})
@@ -88,6 +98,8 @@ macro(CMAKE_FORCE_CXX_COMPILER compiler id)
 endmacro()
 
 macro(CMAKE_FORCE_Fortran_COMPILER compiler id)
+  message(DEPRECATION "The CMAKE_FORCE_Fortran_COMPILER macro is deprecated.  "
+    "Instead just set CMAKE_Fortran_COMPILER and allow CMake to identify the compiler.")
   set(CMAKE_Fortran_COMPILER "${compiler}")
   set(CMAKE_Fortran_COMPILER_ID_RUN TRUE)
   set(CMAKE_Fortran_COMPILER_ID ${id})