CMakeLists.txt: enable building Git using LLVM/Clang on Windows

While Clang is available on Windows, it uses the Visual Studio linker by default:

"For Windows projects, Visual Studio by default invokes Clang in clang-cl mode. It links with the Microsoft implementation of the Standard Library."

https://docs.microsoft.com/en-us/cpp/build/clang-support-msbuild?view=msvc-160

Since we want to build with the Clang/llvm-mingw toolchain, we have to get rid of some assumptions in the CMakeLists.txt file, as it's basically a GNU compiler and linker.

Signed-off-by: Dennis Ameling <dennis@dennisameling.com>

contrib/buildsystems/CMakeLists.txt

@@ -280,7 +280,8 @@ endif()
 
 #Platform Specific
 if(CMAKE_SYSTEM_NAME STREQUAL "Windows")
-	if(CMAKE_C_COMPILER_ID STREQUAL "MSVC" OR CMAKE_C_COMPILER_ID STREQUAL "Clang")
+	# MinGW Makefiles is used by the llvm-mingw toolchain on Windows (Clang)
+	if(CMAKE_C_COMPILER_ID STREQUAL "MSVC" OR (NOT CMAKE_GENERATOR STREQUAL "MinGW Makefiles" AND CMAKE_C_COMPILER_ID STREQUAL "Clang"))
 		include_directories(${CMAKE_SOURCE_DIR}/compat/vcbuild/include)
 		add_compile_definitions(_CRT_SECURE_NO_WARNINGS _CRT_NONSTDC_NO_DEPRECATE)
 	endif()
@@ -719,7 +720,8 @@ endif()
 if(WIN32)
 	target_link_libraries(common-main ws2_32 ntdll ${CMAKE_BINARY_DIR}/git.res)
 	add_dependencies(common-main git-rc)
-	if(CMAKE_C_COMPILER_ID STREQUAL "GNU")
+	# MinGW Makefiles is used by the llvm-mingw toolchain on Windows (Clang)
+	if(CMAKE_C_COMPILER_ID STREQUAL "GNU" OR (CMAKE_GENERATOR STREQUAL "MinGW Makefiles" AND CMAKE_C_COMPILER_ID STREQUAL "Clang"))
 		target_link_options(common-main PUBLIC -municode -Wl,--nxcompat -Wl,--dynamicbase -Wl,--pic-executable,-e,mainCRTStartup)
 	elseif(CMAKE_C_COMPILER_ID STREQUAL "Clang")
 		target_link_options(common-main PUBLIC -municode -Wl,-nxcompat -Wl,-dynamicbase -Wl,-entry:wmainCRTStartup -Wl,invalidcontinue.obj)