the subject says it all. 

error message looks like this:
(this one is from from Min Sik Kims bulk build[1])

gcc -shared  .libs/array.o .libs/builtin_hashes.o .libs/conf.o
.libs/debug.o .libs/dlinked_list.o .libs/file.o .libs/linked_list.o
.libs/mem.o .libs/msgs.o .libs/obj.o .libs/objpair.o .libs/options.o
.libs/regexp.o .libs/socket.o .libs/str.o .libs/strings.o
.libs/snprintf.o .libs/tok.o .libs/url.o  -Wl,--rpath
-Wl,/usr/pkgsrc/devel/libast/work/.buildlink/lib -Wl,--rpath
-Wl,/usr/pkgsrc/devel/libast/work/.buildlink/lib
-L/usr/pkgsrc/devel/libast/work/.buildlink/lib
/usr/pkgsrc/devel/libast/work/.buildlink/lib/libImlib2.so
/usr/pkgsrc/devel/libast/work/.buildlink/lib/libfreetype.so -lz -lX11
-lXext -ldl -lm /usr/pkgsrc/devel/libast/work/.buildlink/lib/libpcre.so
-Wl,-R/usr/pkg/lib -Wl,--rpath -Wl,/usr/pkg/lib -Wl,-soname
-Wl,libast.so.2 -o .libs/libast.so.2.0.1
/usr/bin/ld: cannot find -lX11
collect2: ld returned 1 exit status

--
--- Makefile.orig       2005-01-11 15:00:31.000000000 +0100
+++ Makefile    2005-02-05 03:09:55.000000000 +0100
@@ -12,6 +12,7 @@
 USE_LIBTOOL=           YES
 GNU_CONFIGURE=         YES
 TEST_TARGET=           test
+USE_X11=               YES
 
 .include "../../mk/bsd.prefs.mk"
 
--

   -- Sebastian Prause


[1] ftp://ftp.netbsd.org/pub/NetBSD/misc/minskim/pkgstat/Linux-i686/20050128.0224/devel/libast/.broken.html