CMakeLists: Generate Dawn headers separately from libdawn

libdawn will be one of the libraries produced but other libraries like
libdawn_native don't need to link against it. However they do need the
Dawn headers so we generate them separately.

This also makes all internal targets depend on the header generation and
have the include directories necessary for those headers.

Also has a small fix for setting compile flags only for C++ files.

src/common/CMakeLists.txt

@@ -34,5 +34,4 @@ list(APPEND COMMON_SOURCES
 )
 
 add_library(dawn_common STATIC ${COMMON_SOURCES})
-target_include_directories(dawn_common PUBLIC ${SRC_DIR})
 DawnInternalTarget("" dawn_common)