Also just copy libgit2 dll for its install target

since otherwise it puts pkg-config files in LIB_INSTALL_DIR
which we don't need - if mbedtls separates runtime from library
destinations then we could separate BIN_INSTALL_DIR from LIB_INSTALL_DIR
or convince libgit2 to use the standard GNUInstallDirs names

deps/libgit2.mk

@@ -67,7 +67,7 @@ endif
 	echo 1 > $@
 
 $(LIBGIT2_OBJ_TARGET): $(LIBGIT2_OBJ_SOURCE) | $(build_shlibdir)
-ifeq ($(BUILD_OS),WINNT)
+ifeq ($(OS),WINNT)
 	cp $< $@
 else
 	$(call make-install,$(LIBGIT2_SRC_DIR),)