Compile without -Werror when compiling with clang

clang currently prints a lot of warnings that will be fixed in
subsequent commits

CMakeLists.txt

@@ -436,7 +436,7 @@ if("Ninja" STREQUAL ${CMAKE_GENERATOR})
 endif()
 
 # Add -Werror to C/C++ flags for newer compilers
-if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.9)
+if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.9 AND NOT CMAKE_CXX_COMPILER_ID MATCHES "Clang")
   set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Werror")
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Werror")
 endif()