Fix lcov for newer llvm based gcov

lcov is broken on newer versions of OSX where the version of gcov is now pointing at an llvm implementation.  This does not have the -v commandline parameter and causes lcov to be incorrectly built and consequently always fail.

The code comes from http://stackoverflow.com/questions/22343725/xcode-5-1-unit-test-coverage-analysis-fails-on-files-using-blocks - I just thought it would be handy to merge it into homebrew so it works ;)

Tested on 10.9 with Xcode 5.1 - then tested with code coverage of a small library.