[CMake] Warn when overriding CMAKE_DISABLE_PRECOMPILED_HEADERS

Signed-off-by: Kajetan Puchalski <[email protected]>

Author: mrkajetanp

llvm/CMakeLists.txt

@@ -288,7 +288,11 @@ if(LLVM_CCACHE_BUILD)
     else()
       # Until a way to reliably configure ccache on Windows is found,
       # disable precompiled headers for Windows + ccache builds
-      set(CMAKE_DISABLE_PRECOMPILE_HEADERS "ON")
+      if(NOT CMAKE_DISABLE_PRECOMPILE_HEADERS)
+        message(WARNING "Using ccache with precompiled headers on Windows is currently not supported.
+          CMAKE_DISABLE_PRECOMPILE_HEADERS will be set to ON.")
+        set(CMAKE_DISABLE_PRECOMPILE_HEADERS "ON")
+      endif()
       if(LLVM_CCACHE_MAXSIZE OR LLVM_CCACHE_DIR OR
          NOT LLVM_CCACHE_PARAMS MATCHES "CCACHE_CPP2=yes CCACHE_HASHDIR=yes CCACHE_SLOPPINESS=pch_defines,time_macros")
         message(FATAL_ERROR "Ccache configuration through CMake is not supported on Windows. Please use environment variables.")