Added formula for Apache's benchmark tool 'ab'

"ab" on Lion (and Mountain Lion) has many issues handling its sockets, see for example:
http://superuser.com/questions/323840/

This formula uses the latest Apache httpd-2.4.2 tarball, because that comes with a "configure" script, as opposed to the Subversion trunk.

The formula patches the source of "ab" (one file, "support/ab.c") to fix a bug that causes connections to IPv4 hosts to fail. The result is an "ab" that is identical to the current Subversion trunk.
As soon as there is a new tarball (presumably 2.4.3), this patch won't be necessary anymore.

Finally, "configure" is also patched to do away with the PCRE requirement, because "ab" does not use that.
Tested on 10.7.4 with Xcode 4.3.3 and 10.8 DP4 with Xcode 4.5 DP2.

I have written a more detailed description of the [bugs in ab on Lion and how they are fixed](http://simon.heimlicher.com/articles/2012/07/08/fix-apache-bench-ab-on-os-x-lion) by the formula.
This is my first formula, please let me know if there are ways to improve it.