cmake: remove -Werror from CMAKE_C_FLAGS

This should have been removed since we never intend to test on all compilers.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index ce367ed..f2b5483 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -12,7 +12,7 @@
 
 if (CMAKE_COMPILER_IS_GNUCC)
     set(COMMON_COMPILE_FLAGS "-Wall -Wextra -Wno-sign-compare -Wno-unused-parameter -Wno-missing-field-initializers")
-    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=c99 ${COMMON_COMPILE_FLAGS} -Werror")
+    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=c99 ${COMMON_COMPILE_FLAGS}")
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${COMMON_COMPILE_FLAGS}")
     if (UNIX)
         set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -fvisibility=hidden")