CMake: Replace CMAKE_COMPILER_IS_MINGW by MINGW

See https://discourse.cmake.org/t/cmake-compiler-is-mingw-removed/4539 and https://cmake.org/cmake/help/latest/variable/MINGW.html

CMakeLists.txt

@@ -246,7 +246,7 @@ IF( UNIX )
 ENDIF()
 
 # Grouped compiler settings ########################################
-IF ((CMAKE_C_COMPILER_ID MATCHES "GNU") AND NOT CMAKE_COMPILER_IS_MINGW)
+IF ((CMAKE_C_COMPILER_ID MATCHES "GNU") AND NOT MINGW)
   IF(NOT ASSIMP_HUNTER_ENABLED)
     SET(CMAKE_CXX_STANDARD 11)
     SET(CMAKE_POSITION_INDEPENDENT_CODE ON)
@@ -283,7 +283,7 @@ ELSEIF (CMAKE_CXX_COMPILER_ID MATCHES "Clang" )
   ENDIF()
   SET(CMAKE_CXX_FLAGS "-fvisibility=hidden -fno-strict-aliasing -Wall -Wno-long-long ${CMAKE_CXX_FLAGS}" )
   SET(CMAKE_C_FLAGS "-fno-strict-aliasing ${CMAKE_C_FLAGS}")
-ELSEIF( CMAKE_COMPILER_IS_MINGW )
+ELSEIF( MINGW )
   IF (CMAKE_CXX_COMPILER_VERSION VERSION_LESS 7.0)
     message(FATAL_ERROR "MinGW is too old to be supported. Please update MinGW and try again.")
   ELSEIF(CMAKE_CXX_COMPILER_VERSION VERSION_LESS 7.3)

code/CMakeLists.txt

@@ -557,7 +557,7 @@ ADD_ASSIMP_IMPORTER( IFC
 if (ASSIMP_BUILD_IFC_IMPORTER)
   if (MSVC)
     set_source_files_properties(Importer/IFC/IFCReaderGen1_2x3.cpp Importer/IFC/IFCReaderGen2_2x3.cpp PROPERTIES COMPILE_FLAGS "/bigobj")
-  elseif(CMAKE_COMPILER_IS_MINGW)
+  elseif(MINGW)
     set_source_files_properties(Importer/IFC/IFCReaderGen1_2x3.cpp Importer/IFC/IFCReaderGen2_2x3.cpp PROPERTIES COMPILE_FLAGS "-O2 -Wa,-mbig-obj")
   endif()
 endif ()
@@ -862,7 +862,7 @@ ADD_ASSIMP_IMPORTER( MMD
 # optimizations that take up extra space. Given that the issue is a string table overflowing, -Os seemed appropriate
 # Also, I'm not positive if both link & compile flags are needed, but this hopefully ensures that the issue should not
 # recur for edge cases such as static builds.
-if ((CMAKE_COMPILER_IS_MINGW) AND (CMAKE_BUILD_TYPE MATCHES Debug))
+if ((MINGW) AND (CMAKE_BUILD_TYPE MATCHES Debug))
   message("-- Applying MinGW StepFileGen1.cpp Debug Workaround")
   SET_SOURCE_FILES_PROPERTIES(Importer/StepFile/StepFileGen1.cpp PROPERTIES COMPILE_FLAGS -Os )
   SET_SOURCE_FILES_PROPERTIES(Importer/StepFile/StepFileGen1.cpp PROPERTIES LINK_FLAGS -Os )

contrib/android-cmake/android.toolchain.cmake

@@ -1616,6 +1616,7 @@ endif()
 if( CMAKE_GENERATOR MATCHES "Ninja" AND CMAKE_HOST_WIN32 )
  # it is a bad hack after all
  # CMake generates Ninja makefiles with UNIX paths only if it thinks that we are going to build with MinGW
+ set( MINGW TRUE ) # tell CMake that we are MinGW
  set( CMAKE_COMPILER_IS_MINGW TRUE ) # tell CMake that we are MinGW
  set( CMAKE_CROSSCOMPILING TRUE )    # stop recursion
  enable_language( C )