Merge topic 'compiler-id-msvc-has-include' into release-3.21

c6aaaf06 DetermineCompiler: Restore identification of MSVC with no INCLUDE dirs

Acked-by: Kitware Robot <kwrobot@kitware.com>
Acked-by: Ben Boeckel <ben.boeckel@kitware.com>
Merge-request: !6284

Modules/CMakeDetermineCCompiler.cmake

@@ -85,6 +85,10 @@ else()
 
     # ARMClang need target options
     "--target=arm-arm-none-eabi -mcpu=cortex-m3"
+
+    # MSVC needs at least one include directory for __has_include to function,
+    # but custom toolchains may run MSVC with no INCLUDE env var and no -I flags.
+    "-I__does_not_exist__"
     )
 endif()
 if(CMAKE_C_COMPILER_TARGET)

Modules/CMakeDetermineCXXCompiler.cmake

@@ -80,6 +80,10 @@ else()
 
     # ARMClang need target options
     "--target=arm-arm-none-eabi -mcpu=cortex-m3"
+
+    # MSVC needs at least one include directory for __has_include to function,
+    # but custom toolchains may run MSVC with no INCLUDE env var and no -I flags.
+    "-I__does_not_exist__"
     )
 endif()