allow building with external libraries

[ghost]

To package pfff (for usage by the hhvm hackificactor) for debian and fedora, we will need to be able to build with the system libraries.

[phooji]

@jrobeson Could you elaborate a little -- what system libraries are you referring to?

[ghost]

any of the libraries bundled in /external. They don't seem to be able to be overridden by versions that exist on the system.

[phooji]

@jrobeson Happy to accept a pull request; not something I have time for myself, unfortunately.

[ghost]

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