CMake: Enable support for clang with gnu-like cmdline on Windows

docs/CMake API.md

@@ -137,6 +137,22 @@ Note that the static library produced by `juce_add_binary_data` automatically se
 Building universal binaries that will run on both arm64 and x86_64 can be achieved by
 configuring the CMake project with `"-DCMAKE_OSX_ARCHITECTURES=arm64;x86_64"`.
 
+### Building with Clang on Windows
+
+Clang-cl (Clang with MSVC-like command-line) should work by default. If you are generating a Visual
+Studio project, and have installed the LLVM package which is distributed with Visual Studio, then
+you can configure a Clang-cl build by passing "-T ClangCL" on your configuration commandline.
+
+If you wish to use Clang with GNU-like command-line instead, you can pass
+`-DCMAKE_CXX_COMPILER=clang++` and `-DCMAKE_C_COMPILER=clang` on your configuration commandline.
+clang++ and clang must be on your `PATH` for this to work. Only more recent versions of CMake
+support Clang's GNU-like command-line on Windows. CMake 3.12 is not supported, CMake 3.15 has
+support, CMake 3.20 or higher is recommended.  Note that CMake doesn't seem to automatically link a
+runtime library when building in this configuration, but this can be remedied by setting the
+`MSVC_RUNTIME_LIBRARY` property. See the [official
+documentation](https://cmake.org/cmake/help/v3.15/prop_tgt/MSVC_RUNTIME_LIBRARY.html) of this
+property for usage recommendations.
+
 ### A note about compile definitions
 
 Module options and plugin options that would previously have been set in the Projucer can be set on

extras/Build/CMake/JUCEHelperTargets.cmake

@@ -1,7 +1,7 @@
 add_library(juce_recommended_warning_flags INTERFACE)
 add_library(juce::juce_recommended_warning_flags ALIAS juce_recommended_warning_flags)
 
-if((CMAKE_CXX_COMPILER_ID STREQUAL "MSVC") OR (CMAKE_CXX_SIMULATE_ID STREQUAL "MSVC"))
+if((CMAKE_CXX_COMPILER_ID STREQUAL "MSVC") OR (CMAKE_CXX_COMPILER_FRONTEND_VARIANT STREQUAL "MSVC"))
     target_compile_options(juce_recommended_warning_flags INTERFACE "/W4")
 elseif((CMAKE_CXX_COMPILER_ID STREQUAL "Clang") OR (CMAKE_CXX_COMPILER_ID STREQUAL "AppleClang"))
     target_compile_options(juce_recommended_warning_flags INTERFACE
@@ -33,7 +33,7 @@ endif()
 add_library(juce_recommended_config_flags INTERFACE)
 add_library(juce::juce_recommended_config_flags ALIAS juce_recommended_config_flags)
 
-if((CMAKE_CXX_COMPILER_ID STREQUAL "MSVC") OR (CMAKE_CXX_SIMULATE_ID STREQUAL "MSVC"))
+if((CMAKE_CXX_COMPILER_ID STREQUAL "MSVC") OR (CMAKE_CXX_COMPILER_FRONTEND_VARIANT STREQUAL "MSVC"))
     target_compile_options(juce_recommended_config_flags INTERFACE
         $<IF:$<CONFIG:Debug>,/Od,/Ox> $<$<STREQUAL:"${CMAKE_CXX_COMPILER_ID}","MSVC">:/MP> /EHsc)
 elseif((CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
@@ -49,7 +49,7 @@ endif()
 add_library(juce_recommended_lto_flags INTERFACE)
 add_library(juce::juce_recommended_lto_flags ALIAS juce_recommended_lto_flags)
 
-if((CMAKE_CXX_COMPILER_ID STREQUAL "MSVC") OR (CMAKE_CXX_SIMULATE_ID STREQUAL "MSVC"))
+if((CMAKE_CXX_COMPILER_ID STREQUAL "MSVC") OR (CMAKE_CXX_COMPILER_FRONTEND_VARIANT STREQUAL "MSVC"))
     target_compile_options(juce_recommended_lto_flags INTERFACE
         $<$<CONFIG:Release>:$<IF:$<STREQUAL:"${CMAKE_CXX_COMPILER_ID}","MSVC">,-GL,-flto>>)
     target_link_libraries(juce_recommended_lto_flags INTERFACE

extras/Build/CMake/JUCEUtils.cmake

@@ -670,7 +670,7 @@ function(juce_add_module module_path)
 
         _juce_link_libs_from_metadata("${module_name}" "${metadata_dict}" linuxLibs)
     elseif(CMAKE_SYSTEM_NAME STREQUAL "Windows")
-        if((CMAKE_CXX_COMPILER_ID STREQUAL "MSVC") OR (CMAKE_CXX_SIMULATE_ID STREQUAL "MSVC"))
+        if((CMAKE_CXX_COMPILER_ID STREQUAL "MSVC") OR (CMAKE_CXX_COMPILER_FRONTEND_VARIANT STREQUAL "MSVC"))
             if(module_name STREQUAL "juce_gui_basics")
                 target_compile_options(${module_name} INTERFACE /bigobj)
             endif()