Make sure configure aborts if libjpeg isn't found.


git-svn-id: svn://svn.code.sf.net/p/tigervnc/code/trunk@4311 3789f03b-4d11-0410-bbf8-ca57d06f2519
diff --git a/configure.ac b/configure.ac
index defe262..64c0b7a 100644
--- a/configure.ac
+++ b/configure.ac
@@ -195,7 +195,7 @@
 AC_CONFIG_SUBDIRS([zlib])
 
 dnl Check for libjpeg library
-AC_SEARCH_LIBS([jpeg_destroy_compress], [jpeg])
+AC_SEARCH_LIBS([jpeg_destroy_compress], [jpeg], [], [AC_MSG_ERROR(["libjpeg is required to build TigerVNC"])])
 
 AC_CHECK_FUNCS([vsnprintf snprintf strcasecmp strncasecmp])