Backport #654 from trunk:
Add a security patch for MPFR-2.3.1.

needed to complete #657 from trunk (already backported as #658):
Build a thread-safe MPFR library.

 # Compute x86_64-specific values

 CT_DoArchValues() {

     # The architecture part of the tuple:

     CT_TARGET_ARCH="${CT_ARCH}"

 }