Set PIC if Hardened GCC is detected.

Under Hardened Gentoo, Phobos and Druntime need to be compiled with PIC=1, otherwise they cannot be linked.
Discussed in more detail [here](http://forum.dlang.org/thread/nqoogrjpqdbmxiaxfihi@forum.dlang.org#post-hekvjmfokatygccxvzmi:40forum.dlang.org).