build: add and use C_STANDARD in depends

depends/hosts/mingw32.mk

@@ -2,15 +2,15 @@ ifneq ($(shell $(SHELL) $(.SHELLFLAGS) "command -v $(host)-g++-posix"),)
 mingw32_CXX := $(host)-g++-posix
 endif
 
-mingw32_CFLAGS=-pipe
+mingw32_CFLAGS=-pipe -std=$(C_STANDARD)
+mingw32_CXXFLAGS=-pipe
 
 ifneq ($(LTO),)
 mingw32_CFLAGS += -flto
+mingw32_CXXFLAGS += -flto
 mingw32_LDFLAGS += -flto
 endif
 
-mingw32_CXXFLAGS=$(mingw32_CFLAGS)
-
 mingw32_release_CFLAGS=-O2
 mingw32_release_CXXFLAGS=$(mingw32_release_CFLAGS)