Use explict libtool CC tag

(sometimes libtool can't decide what tag is correct one
if omited).

Preserve CFLAGS/CPPFLAGS to allow:
CPPFLAGS="-I$HOME/here-is-xfsprogs-installed/include" \
LDFLAGS="-L$HOME/here-is-xfsprogs-installed/lib" \
./configure  ...

Signed-off-by: Arkadiusz Miśkiewicz <arekm@maven.pl>
Reviewed-by: Eric Sandeen <sandeen@sandeen.net>

include/builddefs.in

@@ -11,6 +11,8 @@ DEBUG = @debug_build@
 OPTIMIZER = @opt_build@
 MALLOCLIB = @malloc_lib@
 LOADERFLAGS = @LDFLAGS@
+CFLAGS = @CFLAGS@
+CPPFLAGS = @CPPFLAGS@
 
 LIBXFS = @libxfs@
 LIBACL = @libacl@
@@ -75,7 +77,7 @@ ifeq ($(PKG_PLATFORM),freebsd)
 DEPENDFLAGS = -D__FreeBSD__
 endif
 
-GCFLAGS = $(OPTIMIZER) $(DEBUG) \
+GCFLAGS = $(OPTIMIZER) $(DEBUG) $(CPPFLAGS) \
 	-I$(TOPDIR)/include -DVERSION=\"$(PKG_VERSION)\"
 
 # Global, Platform, Local CFLAGS