use := instead of = for variables and make sure zlib builds with target cflags

package/zlib/zlib.mk

@@ -3,13 +3,13 @@
 # zlib
 #
 #############################################################
-ZLIB_VER=1.2.3
-ZLIB_SOURCE=zlib-$(ZLIB_VER).tar.bz2
-ZLIB_SITE=http://$(BR2_SOURCEFORGE_MIRROR).dl.sourceforge.net/sourceforge/libpng
-ZLIB_DIR=$(BUILD_DIR)/zlib-$(ZLIB_VER)
-ZLIB_CFLAGS= $(TARGET_CFLAGS) -fPIC
+ZLIB_VER:=1.2.3
+ZLIB_SOURCE:=zlib-$(ZLIB_VER).tar.bz2
+ZLIB_SITE:=http://$(BR2_SOURCEFORGE_MIRROR).dl.sourceforge.net/sourceforge/libpng
+ZLIB_DIR:=$(BUILD_DIR)/zlib-$(ZLIB_VER)
+ZLIB_CFLAGS:=$(TARGET_CFLAGS) -fPIC
 ifeq ($(BR2_LARGEFILE),y)
-ZLIB_CFLAGS+= -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64
+ZLIB_CFLAGS+=-D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64
 endif
 
 $(DL_DIR)/$(ZLIB_SOURCE):
@@ -23,6 +23,7 @@ $(ZLIB_DIR)/.source: $(DL_DIR)/$(ZLIB_SOURCE)
 $(ZLIB_DIR)/.configured: $(ZLIB_DIR)/.source
 	(cd $(ZLIB_DIR); \
 		$(TARGET_CONFIGURE_OPTS) \
+		CFLAGS="$(ZLIB_CFLAGS)" \
 		./configure \
 		--shared \
 		--prefix=/usr \