Fixed bug linking to system tpls.

[corbett5]

No description provided.

[TotoGaz]

@corbett5 I'd like to test it against GEOS-DEV/thirdPartyLibs#142
We could include the code (commented), that could be a nice test when deep modifications in the build system are made.

[corbett5]

@TotoGaz I'm not sure what you mean include the code (commented), this doesn't prohibit you from using system libraries, but if you want to use them you'll have to hard-code the path, which seems pretty reasonable to me. Either way would you like me to hold off merging this until you're ready (until you approve this PR)?

[TotoGaz]

I mean it could be interesting to effectively validate our script against all the system libs that are in ubuntu (e.g.). There may be some libs we have forgotten.
Furthermore, from time to time when we move our build system, uncommenting the code installing all the system libs could save us time.

[corbett5]

Ahhh. Have you checked that the versions we use all have packages?