On FreeBSD, htp.pc is not installed correctly

It's placed in /usr/local/lib/pkgconfig/, but pkg-config expects it in /usr/local/libdata/pkgconfig/. The location is currently hard-coded in Makefile.am to $(libdir)/pkgconfig. This is something we should ideally detect when configuring. Alternatively, a configure switch could be added for the user to indicate the correct location.
