Added -fPIC option to configure.ac

Needed when compiling the object code into a library for use in third party tools. 

From man gcc:

If supported for the target machine, emit position-independent code, suitable for dynamic linking and avoiding any limit on the size of the global offset table. This option makes a difference on the m68k, PowerPC and SPARC. Position-independent code requires special support, and therefore works only on certain machines.