do not use -L$Z_DIR/lib when Z_DIR isn't actually set

--- a/configure.ac
+++ b/configure.ac
@@ -392,7 +392,9 @@ if test "$with_zlib" = "no"; then
 else
     AC_CHECK_HEADERS(zlib.h,
         [SAVE_LDFLAGS="${LDFLAGS}"
-	 LDFLAGS="-L${Z_DIR}/lib"
+	if test "x${Z_DIR}" != "x"; then
+	    LDFLAGS="${LDFLAGS} -L${Z_DIR}/lib"
+	fi
 	AC_CHECK_LIB(z, gzread,[
 	    AC_DEFINE([HAVE_LIBZ], [1], [Have compression library])
 	    WITH_ZLIB=1
