Don't break configure if x11 is not found

Author: Erlend E. Aasland

configure

@@ -3522,9 +3522,11 @@ AS_VAR_IF([found_tcltk], [no], [
 dnl FreeBSD has an X11 dependency which is not implicitly resolved.
 AS_CASE([$ac_sys_system],
   [FreeBSD*], [
-    PKG_CHECK_MODULES([X11], [x11], [
-      TCLTK_CFLAGS="$TCLTK_CFLAGS $X11_CFLAGS"
-      TCLTK_LIBS="$TCLTK_LIBS $X11_LIBS"
+    PKG_CHECK_EXISTS([x11], [
+      PKG_CHECK_MODULES([X11], [x11], [
+        TCLTK_CFLAGS="$TCLTK_CFLAGS $X11_CFLAGS"
+        TCLTK_LIBS="$TCLTK_LIBS $X11_LIBS"
+      ])
     ])
   ]
 )