fix lib64 library path (issue #8)

On SUSE (and some other distros), libraries on 64-bit platforms get installed
to ${exec_prefix}/lib64 rather than .../lib.  Detect the correct path by
parsing config.log.

Tested to build successfully on the following:
* openSUSE 12.2 x86_64
* openSUSE 12.1 i586
* CentOS 6.3 x86_64
* CentOS 6.3 i386
* Ubuntu 12.04 x86_64
* Ubuntu 12.04 i386
* OS X 10.8 x86_64