Win32: Fix compilation with standalone LLVM

The /clang: suffix passed to Clang-CL was accidentally also passed to
the regular standalone Clang, which caused compilation to fail.  We now
pass /W3 to Clang-CL, which it interprets as -Wall.

The _CRT_SECURE_NO_WARNINGS macro is now defined for both Clang and
Clang-CL.

The /entry: flag passed to link.exe is now also passed to lld-link,
letting the windows subsystem tests and examples link.

Fixes #1807.
Closes #1824.
Closes #1874.

(cherry picked from commit 061a0263a9)

README.md

@@ -120,6 +120,7 @@ information on what to include when reporting a bug.
 
  - [Win32] Bugfix: `USE_MSVC_RUNTIME_LIBRARY_DLL` had no effect on CMake 3.15 or
    later (#1783,#1796)
+ - [Win32] Bugfix: Compilation with LLVM for Windows failed (#1807,#1824,#1874)
 
 
 ## Contact

examples/CMakeLists.txt

@@ -7,7 +7,8 @@ if (MATH_LIBRARY)
     link_libraries("${MATH_LIBRARY}")
 endif()
 
-if (MSVC)
+# Workaround for the MS CRT deprecating parts of the standard library
+if (MSVC OR CMAKE_C_SIMULATE_ID STREQUAL "MSVC")
     add_definitions(-D_CRT_SECURE_NO_WARNINGS)
 endif()
 
@@ -62,9 +63,13 @@ if (GLFW_USE_OSMESA)
 endif()
 
 if (MSVC)
-    # Tell MSVC to use main instead of WinMain for Windows subsystem executables
+    # Tell MSVC to use main instead of WinMain
     set_target_properties(${GUI_ONLY_BINARIES} PROPERTIES
                           LINK_FLAGS "/ENTRY:mainCRTStartup")
+elseif (CMAKE_C_SIMULATE_ID STREQUAL "MSVC")
+    # Tell Clang using MS CRT to use main instead of WinMain
+    set_target_properties(${GUI_ONLY_BINARIES} PROPERTIES
+                          LINK_FLAGS "-Wl,/entry:mainCRTStartup")
 endif()
 
 if (APPLE)

src/CMakeLists.txt

@@ -121,18 +121,14 @@ if (CMAKE_C_COMPILER_ID STREQUAL "GNU")
 endif()
 
 # Enable a reasonable set of warnings
-if (CMAKE_C_COMPILER_ID STREQUAL "GNU" OR
+# NOTE: The order matters here, Clang-CL matches both MSVC and Clang
+if (MSVC)
+    target_compile_options(glfw PRIVATE "/W3")
+elseif (CMAKE_C_COMPILER_ID STREQUAL "GNU" OR
         CMAKE_C_COMPILER_ID STREQUAL "Clang" OR
         CMAKE_C_COMPILER_ID STREQUAL "AppleClang")
 
-    if (CMAKE_C_SIMULATE_ID STREQUAL "MSVC")
-        # Tell Clang-CL that this is a Clang flag
-        target_compile_options(glfw PRIVATE "/clang:-Wall")
-    else()
     target_compile_options(glfw PRIVATE "-Wall")
-    endif()
-elseif (MSVC)
-    target_compile_options(glfw PRIVATE "/W3")
 endif()
 
 if (WIN32)
@@ -176,7 +172,7 @@ if (BUILD_SHARED_LIBS)
     endif()
 endif()
 
-if (MSVC)
+if (MSVC OR CMAKE_C_SIMULATE_ID STREQUAL "MSVC")
     target_compile_definitions(glfw PRIVATE _CRT_SECURE_NO_WARNINGS)
 endif()
 

tests/CMakeLists.txt

@@ -7,7 +7,8 @@ if (MATH_LIBRARY)
     link_libraries("${MATH_LIBRARY}")
 endif()
 
-if (MSVC)
+# Workaround for the MS CRT deprecating parts of the standard library
+if (MSVC OR CMAKE_C_SIMULATE_ID STREQUAL "MSVC")
     add_definitions(-D_CRT_SECURE_NO_WARNINGS)
 endif()
 
@@ -69,9 +70,13 @@ set_target_properties(${GUI_ONLY_BINARIES} ${CONSOLE_BINARIES} PROPERTIES
                       FOLDER "GLFW3/Tests")
 
 if (MSVC)
-    # Tell MSVC to use main instead of WinMain for Windows subsystem executables
+    # Tell MSVC to use main instead of WinMain
     set_target_properties(${GUI_ONLY_BINARIES} PROPERTIES
                           LINK_FLAGS "/ENTRY:mainCRTStartup")
+elseif (CMAKE_C_SIMULATE_ID STREQUAL "MSVC")
+    # Tell Clang using MS CRT to use main instead of WinMain
+    set_target_properties(${GUI_ONLY_BINARIES} PROPERTIES
+                          LINK_FLAGS "-Wl,/entry:mainCRTStartup")
 endif()
 
 if (APPLE)