Add support for CMake substitutions to USES=pathfix
CMake projects mostly have trouble with where to put pkgconfig data, so I've added some of the most popular cases to substitute for. There are a few projects that use some custom variables that I did not add, but if there are more generic substitutions that should be added to the list, I'm open to suggestions.
Special note for devel/libical:
I got rid of the useless dependency on gmake and a patch for configure (it uses CMake)