Don't silently disable WiFi on Linux if we can't find iwlib.h. (Bug 503597) r=dougt,bsmedberg

configure.in

@@ -7957,6 +7957,8 @@ if test "$NECKO_WIFI"; then
     AC_CHECK_HEADER([iwlib.h])
     if test "$ac_cv_header_iwlib_h" = "yes"; then
       NECKO_WIFI=1
+    else
+      AC_MSG_ERROR([Can't find header iwlib.h for Necko WiFi scanning (might be in package iwlib-dev (Ubuntu) or wireless-tools-devel (Fedora)); use --disable-necko-wifi to disable])
     fi
   elif test "$OS_ARCH" = "Darwin"; then
     NECKO_WIFI=1