build: add and use C_STANDARD in depends

depends/hosts/darwin.mk

@@ -109,15 +109,15 @@ darwin_CXX=env -u C_INCLUDE_PATH -u CPLUS_INCLUDE_PATH \
                -Xclang -internal-externc-isystem$(clang_resource_dir)/include \
                -Xclang -internal-externc-isystem$(OSX_SDK)/usr/include
 
-darwin_CFLAGS=-pipe
+darwin_CFLAGS=-pipe -std=$(C_STANDARD)
+darwin_CXXFLAGS=-pipe
 
 ifneq ($(LTO),)
 darwin_CFLAGS += -flto
+darwin_CXXFLAGS += -flto
 darwin_LDFLAGS += -flto
 endif
 
-darwin_CXXFLAGS=$(darwin_CFLAGS)
-
 darwin_release_CFLAGS=-O2
 darwin_release_CXXFLAGS=$(darwin_release_CFLAGS)