Change INSTALLDIR to /opt/pspdev
Add --with-mpc configure option to GCC

Although this shouldn't be necessary according to
https://gcc.gnu.org/install/prerequisites.html
if a mpc directory exists in a subdirectory of our GCC sources,
it does change my build output. The same can be said for GMP and MPFR.

I also thought that these configure options required a path/assignment,
but I guess not. This should probably be looked into.

It has been this way since the project's intial github commit.