Modules: Rename internal platform-specific compiler determination modules

Rename Modules/Platform/<os>-<lang>.cmake files to
Modules/Platform/<os>-Determine-<lang>.cmake to clarify their role.
For compatibility with user-provided modules, load the old names
if they exist.

Modules/CMakeDetermineCCompiler.cmake

@@ -34,6 +34,7 @@
 include(${CMAKE_ROOT}/Modules/CMakeDetermineCompiler.cmake)
 
 # Load system-specific compiler preferences for this language.
+include(Platform/${CMAKE_SYSTEM_NAME}-Determine-C OPTIONAL)
 include(Platform/${CMAKE_SYSTEM_NAME}-C OPTIONAL)
 if(NOT CMAKE_C_COMPILER_NAMES)
   set(CMAKE_C_COMPILER_NAMES cc)

Modules/CMakeDetermineCXXCompiler.cmake

@@ -33,6 +33,7 @@
 include(${CMAKE_ROOT}/Modules/CMakeDetermineCompiler.cmake)
 
 # Load system-specific compiler preferences for this language.
+include(Platform/${CMAKE_SYSTEM_NAME}-Determine-CXX OPTIONAL)
 include(Platform/${CMAKE_SYSTEM_NAME}-CXX OPTIONAL)
 if(NOT CMAKE_CXX_COMPILER_NAMES)
   set(CMAKE_CXX_COMPILER_NAMES CC)

Modules/CMakeDetermineFortranCompiler.cmake

@@ -20,6 +20,7 @@
 # as a default compiler
 
 include(${CMAKE_ROOT}/Modules/CMakeDetermineCompiler.cmake)
+include(Platform/${CMAKE_SYSTEM_NAME}-Determine-Fortran OPTIONAL)
 include(Platform/${CMAKE_SYSTEM_NAME}-Fortran OPTIONAL)
 if(NOT CMAKE_Fortran_COMPILER_NAMES)
   set(CMAKE_Fortran_COMPILER_NAMES f95)