squashfs-tools: Pass down TARGET_CFLAGS, CPPFLAGS and LDFLAGs

This fixes build errors with external toolchains that don't
automatically search for headers in STAGING_DIR:

mipsel-linux-gnu-gcc   -I. -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE
-D_GNU_SOURCE -DCOMP_DEFAULT=\"gzip\" -Wall -DGZIP_SUPPORT   -c -o
gzip_wrapper.o gzip_wrapper.c
gzip_wrapper.c:27:18: fatal error: zlib.h: No such file or directory
 #include <zlib.h>
                  ^
compilation terminated.
make[3]: *** [gzip_wrapper.o] Error 1

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

utils/squashfs-tools/Makefile

@@ -9,7 +9,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=squashfs-tools
 PKG_VERSION:=4.3
-PKG_RELEASE:=3
+PKG_RELEASE:=4
 PKG_LICENSE:=GPL-2.0
 PKG_LICENSE_FILES:=COPYING
 
@@ -69,6 +69,8 @@ endif
 define Build/Compile
 	$(MAKE) -C $(PKG_BUILD_DIR)/squashfs-tools \
 		CC="$(TARGET_CC)" \
+		EXTRA_CFLAGS="$(TARGET_CFLAGS) $(TARGET_CPPFLAGS)" \
+		EXTRA_LDFLAGS="$(TARGET_LDFLAGS)" \
 		$(MAKE_FLAGS) \
 		mksquashfs unsquashfs
 endef