contrib: zlib: Fix prefix replacement on win32

contrib/src/zlib/rules.mak

@@ -25,7 +25,7 @@ zlib: zlib-$(ZLIB_VERSION).tar.gz .sum-zlib
 
 .zlib: zlib
 ifdef HAVE_WIN32
-	cd $< && $(HOSTVARS) $(MAKE) -fwin32/Makefile.gcc install $(HOSTVARS) $(ZLIB_CONFIG_VARS) LD="$(CC)" INCLUDE_PATH="$(PREFIX)/include" LIBRARY_PATH="$(PREFIX)/lib" BINARY_PATH="$(PREFIX)/bin"
+	cd $< && $(HOSTVARS) $(MAKE) -fwin32/Makefile.gcc install $(HOSTVARS) $(ZLIB_CONFIG_VARS) LD="$(CC)" prefix="$(PREFIX)" INCLUDE_PATH="$(PREFIX)/include" LIBRARY_PATH="$(PREFIX)/lib" BINARY_PATH="$(PREFIX)/bin"
 else
 	cd $< && $(HOSTVARS_PIC) $(ZLIB_CONFIG_VARS) ./configure --prefix=$(PREFIX) --static
 	cd $< && $(MAKE) install