Fix compiler detection and flags

- Use CMAKE_<LANG>_COMPILER_ID to detect compiler as CMake docs suggest
- Support clang properly
- Don't set -Werror

CMakeLists.txt

@@ -843,10 +843,10 @@ endif()
 
 # Stratagus needs to have char by default signed
 # No idea how to tell this to other compilers
-if(CMAKE_COMPILER_IS_GNUCXX)
-	set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fsigned-char -Werror")
+if(CMAKE_CXX_COMPILER_ID MATCHES "GNU|Clang")
+	set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fsigned-char")
 endif()
-if(CMAKE_COMPILER_IS_GNUC)
+if(CMAKE_C_COMPILER_ID MATCHES "GNU|Clang")
 	set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -fsigned-char")
 endif()
 if(WIN32 AND MSVC)