fix the clang problem when building coq

The problem is with the option -fno-defer-pop, which is understood
by gcc but not by clang under Mac OS X.

This deals with issue https://github.com/HoTT/HoTT/issues/341

A few other problems are fixed, too, notably with sed in the
script "configure", and with the script not stopping on errors.

NOTE: If you are using a version of ocamlc installed by homebrew and are running 
Mavericks, then execute these commands:

    brew update
    brew upgrade objective-caml

More extreme, if that doesn't work, is to follow it with

    brew remove objective-caml
    brew install objective-caml


(See https://github.com/Homebrew/homebrew/pull/27611 for the discussion about the homebrew upgrade.)