Use pkg-config first, then icu-config if it fails.

When trying to determine cflags & libs related to ICU, first use pkg-config
and only use icu-config if this fails.

On distributions were a multiarch libicu-dev package is available, this makes
it possible to build both a 32 bit & 64 bit version of PHP with --enable-intl
without the need for chroots.

Note: for now, Debian does not provide such a package (for more information,
see http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=699763).