Require C11

This required silencing C5105 which emits warnings about system
headers which we can't change.

https://learn.microsoft.com/en-us/cpp/error-messages/compiler-warnings/c5105?view=msvc-170

cmake/Macros.cmake

@@ -19,8 +19,8 @@ macro(csfml_add_library target)
     # add warnings
     set_file_warnings(${THIS_SOURCES})
 
-    # require C99 and C++17
-    target_compile_features(${target} PUBLIC c_std_99 cxx_std_17)
+    # require C11 and C++17
+    target_compile_features(${target} PUBLIC c_std_11 cxx_std_17)
 
     # define the export symbol of the module
     string(REPLACE "-" "_" NAME_UPPER "${target}")