FindPETSc: Fixed paths and library names on windows

If PETSc static libs (/MT, static) disagree with existing library flags
(/MD, dynamic), we issue a warning telling the user to choose a
compatible compilation mode.

Windows library paths need not start with "/", requiring more permissive
regex patterns.

Squashed-by: Jed Brown <jed@59A2.org>
Added . to lib detection regex