Specify icu4c path with --with-icu-dir

This is an attempt to fix #434.

However this will not work until the icu4c bottles are rebuilt. Reason is described [here](https://github.com/mxcl/homebrew/pull/16302#issuecomment-14091130).

I've tested this with icu4c built from source and it works fine.

I haven't tested but I think building icu4c from source can also fix the `--with-intl` problem while building php.