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.

A few other problems are fixed, too, notably with sed in the
script "configure", and with the script not stopping on errors.
improve the install script

... by making the two sed edits independent.

Also reduce the verbosty of the build.  Although, for remote debugging
it can be useful.
Merge pull request #342 from DanGrayson/pull-fix-mac-os-x-clang-option-problem

fix the clang problem when building coq