Support compilation with standalone mozjs185 release

Adjust the detection logic so that we look for mozjs-185.pc first.  If
we have this, we can skip all kinds of insanity.

See https://bugzilla.mozilla.org/show_bug.cgi?id=628723
for the discussion about creating this release.

https://bugzilla.gnome.org/show_bug.cgi?id=646369