gpg-agent: Set correct stdint.h header path for Mac OSX Mavericks

Mac OSX Mavericks no longer includes headers such as stdint.h in /usr/include, even with the Command Line Tools installed. 

Previous versions of Mac OSX might actually have the stdint.h file using the same check I have currently, but I don't have a system to test that on. If that's so, then we can just use the check I have with that location.