build: support LTO in depends

No Qt for now.

depends/hosts/mingw32.mk

@@ -3,6 +3,12 @@ mingw32_CXX := $(host)-g++-posix
 endif
 
 mingw32_CFLAGS=-pipe
+
+ifneq ($(LTO),)
+mingw32_CFLAGS += -flto
+mingw32_LDFLAGS += -flto
+endif
+
 mingw32_CXXFLAGS=$(mingw32_CFLAGS)
 
 mingw32_release_CFLAGS=-O2