Generalize installing prebuilt libraries

Only libffi for now, libpcre will be added next
Add the libpcre prebuilt library
Merge branch 'master' of github.com:begriffs/heroku-buildpack-ghc into prebuilt-library

Conflicts:
	bin/compile
Put all unwanted files in the temporary build path
Don't build static libraries

Only shared libraries are needed...
Copy libffi files into the correct cache/working paths

See also atgreen/libffi#55 atgreen/libffi#24
Merge branch 'master' of github.com:begriffs/heroku-buildpack-ghc into prebuilt-library

Conflicts:
	bin/compile
Delete unnecessary files from the prebuilt lib/include directories
Handle symbolic links correctly

E.g. shared object (.so) aliases need to be copied with the relative link in place
Disable building the C++ version of pcre
Merge pull request #18 from rehno-lindeque/prebuilt-library

Add pcre library