Give a better error message when zlib.h is not found.

svn:r3618

configure.in

@@ -141,10 +141,12 @@ AC_SYS_LARGEFILE
 
 dnl The warning message here is no longer strictly accurate.
 
-AC_CHECK_HEADERS(unistd.h string.h signal.h netdb.h ctype.h sys/stat.h sys/types.h fcntl.h sys/fcntl.h sys/ioctl.h sys/socket.h sys/time.h netinet/in.h arpa/inet.h errno.h assert.h time.h pwd.h grp.h zlib.h, , AC_MSG_WARN(some headers were not found, compilation may fail))
+AC_CHECK_HEADERS(unistd.h string.h signal.h netdb.h ctype.h sys/stat.h sys/types.h fcntl.h sys/fcntl.h sys/ioctl.h sys/socket.h sys/time.h netinet/in.h arpa/inet.h errno.h assert.h time.h pwd.h grp.h, , AC_MSG_WARN(some headers were not found, compilation may fail))
 
 AC_CHECK_HEADERS(event.h, , AC_MSG_ERROR(Libevent header (event.h) not found. Tor requires libevent to build.))
 
+AC_CHECK_HEADERS(zlib.h, , AC_MSG_ERROR(Zlib header (zlib.h) not found. Tor requires zlib to build. You may need to install a zlib development package.))
+
 dnl These headers are not essential
 
 AC_CHECK_HEADERS(stdint.h sys/types.h inttypes.h sys/param.h sys/wait.h sys/limits.h netinet/in.h arpa/inet.h machine/limits.h syslog.h sys/time.h sys/resource.h pthread.h)