Remove unused CXX_FLAGS variable

The Visual Studio generators apparently don't like the $<COMPILE_LANGUAGE:...>
expression. Since it's not actually used for anything at the moment, remove it.

CMakeLists.txt

@@ -66,7 +66,6 @@ endif()
 SET(CPP_DEFS ) # C pre-process, not C++
 SET(INC_PATHS )
 SET(C_FLAGS )
-SET(CXX_FLAGS )
 SET(LINKER_FLAGS )
 SET(EXTRA_LIBS )
 
@@ -1430,7 +1429,7 @@ CONFIGURE_FILE(
 # Add a static library with common functions used by multiple targets
 ADD_LIBRARY(common STATIC ${COMMON_OBJS})
 TARGET_COMPILE_DEFINITIONS(common PRIVATE ${CPP_DEFS})
-TARGET_COMPILE_OPTIONS(common PRIVATE ${C_FLAGS} $<$<COMPILE_LANGUAGE:CXX>:${CXX_FLAGS}>)
+TARGET_COMPILE_OPTIONS(common PRIVATE ${C_FLAGS})
 
 
 UNSET(HAS_ROUTER)
@@ -1463,7 +1462,7 @@ ELSE()
         ADD_LIBRARY(OpenAL SHARED router/router.cpp router/router.h router/alc.cpp router/al.cpp)
         TARGET_COMPILE_DEFINITIONS(OpenAL
             PRIVATE AL_BUILD_LIBRARY AL_ALEXT_PROTOTYPES ${CPP_DEFS})
-        TARGET_COMPILE_OPTIONS(OpenAL PRIVATE ${C_FLAGS} $<$<COMPILE_LANGUAGE:CXX>:${CXX_FLAGS}>)
+        TARGET_COMPILE_OPTIONS(OpenAL PRIVATE ${C_FLAGS})
         TARGET_LINK_LIBRARIES(OpenAL PRIVATE ${COMMON_LIB} ${LINKER_FLAGS})
         SET_TARGET_PROPERTIES(OpenAL PROPERTIES PREFIX "")
         SET_TARGET_PROPERTIES(OpenAL PROPERTIES OUTPUT_NAME ${LIBNAME})
@@ -1489,7 +1488,7 @@ TARGET_COMPILE_DEFINITIONS(${IMPL_TARGET}
     PRIVATE AL_BUILD_LIBRARY AL_ALEXT_PROTOTYPES ${CPP_DEFS})
 TARGET_INCLUDE_DIRECTORIES(${IMPL_TARGET}
     PRIVATE "${OpenAL_SOURCE_DIR}/OpenAL32/Include" "${OpenAL_SOURCE_DIR}/Alc" ${INC_PATHS})
-TARGET_COMPILE_OPTIONS(${IMPL_TARGET} PRIVATE ${C_FLAGS} $<$<COMPILE_LANGUAGE:CXX>:${CXX_FLAGS}>)
+TARGET_COMPILE_OPTIONS(${IMPL_TARGET} PRIVATE ${C_FLAGS})
 TARGET_LINK_LIBRARIES(${IMPL_TARGET}
     PRIVATE ${COMMON_LIB} ${LINKER_FLAGS} ${EXTRA_LIBS} ${MATH_LIB})
 IF(TARGET build_version)