configure: fix vasprintf test; cosmetics.

The vasprintf test was always failing because it needed stdarg.h.
Misc cosmetics: rename compat objs to COMPAT_OBJS.

lib/Makefile

@@ -44,7 +44,7 @@ OBJS += plist_remove.o plist_fetch.o util.o util_hash.o
 OBJS += repo.o repo_pkgdeps.o repo_sync.o
 OBJS += rpool.o cb_util.o proplib_wrapper.o
 OBJS += package_alternatives.o
-OBJS += $(EXTOBJS) $(COMPAT_SRCS)
+OBJS += $(EXTOBJS) $(COMPAT_OBJS)
 
 .PHONY: all
 all: libxbps.so libxbps.a