Do not hardcode lib path

The changes from 3408b8e5f194774e529e223173d68a1a8f880ea5 cause
regressions in distros which do not use lib as folder for
arch-specific libraries, like Fedora x86_64.

Instead, reloc_libdir should be used (as in data/mono.pc.in).

This change is released under the MIT/X11 license.
Merge pull request #1275 from ranma42/fix-lib64

Fix build with lib64 folder for arch-specific libraries