build: require C11

The atomics support used in libdispatch requires C11.  Ensure that the
compiler supports it and that the support is enabled.

Author: compnerd

CMakeLists.txt

@@ -8,9 +8,13 @@ project(dispatch
         LANGUAGES C CXX)
 enable_testing()
 
-set(CMAKE_C_VISIBILITY_PRESET hidden)
+set(CMAKE_C_STANDARD 11)
+set(CMAKE_C_STANDARD_REQUIRED YES)
+
 set(CMAKE_CXX_STANDARD 11)
 
+set(CMAKE_C_VISIBILITY_PRESET hidden)
+
 set(CMAKE_THREAD_PREFER_PTHREAD TRUE)
 set(THREADS_PREFER_PTHREAD_FLAG TRUE)
 find_package(Threads REQUIRED)