Win32: Fix clang-cl interpreting -Wall as /Wall

Unlike -Wall, VS /Wall really means all warnings.

Closes #1780.

(cherry picked from commit ac627706ef)

README.md

@@ -224,6 +224,7 @@ skills.
  - Eloi Marín Gratacós
  - Stefan Gustavson
  - Jonathan Hale
+ - hdf89shfdfs
  - Sylvain Hellegouarch
  - Matthew Henry
  - heromyth

src/CMakeLists.txt

@@ -125,7 +125,12 @@ if ("${CMAKE_C_COMPILER_ID}" STREQUAL "GNU" OR
     "${CMAKE_C_COMPILER_ID}" STREQUAL "Clang" OR
     "${CMAKE_C_COMPILER_ID}" STREQUAL "AppleClang")
 
-    target_compile_options(glfw PRIVATE "-Wall")
+    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()