Bug 760190: configure.ac should be able to build --with-icu without icu-config tool <https://bugzilla.gnome.org/show_bug.cgi?id=760190> * configure.ac: Add fallback tests if the icu-config tool is not installed (like on Mac OS X). This also allows an include prefix to be set using "--with-icu=/prefix/to/icu", similar to --with-iconv.