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ARROW-1236: Fix lib path in pkg-config file

The `CMAKE_INSTALL_LIBDIR` already contains the prefix on a default
installation (such as in the conda-forge package) and therefore
prepending the prefix results in the wrong path.

While apparently it is not clear what this variable should be
(https://bugzilla.redhat.com/show_bug.cgi?id=795542), it seems that
it's used as an absolute path in the rest of the project.

Author: Zahari <zaharid@gmail.com>

Closes #866 from Zaharid/fixpc and squashes the following commits:

231238c [Zahari] Use better cmake variables
4d1c69a [Zahari] Fix lib path in pkg-config file