math/ntl: Update to 9.11.0

* Updated to latest source: 9.11.0
* Fixed GMP option
  -- Wasn't previously a choice to enable/disable
* Changed CONFIGURE_ARGS
  -- Configure script gave CC=${CC} and NTL_STD_CXX=on is invalid
* Changed post-install-DOCS-on
  -- Should install during install phase, not after