Fixes #219: added configure script for Feel++
 - type <Feel++ toplevel directory>/configure -debug -> Debug mode
 - type <Feel++ toplevel directory>/configure -release -> Release mode
 - type <Feel++ toplevel directory>/configure -help -> for a help message

this script has been initially develop in feelpp/harts by @gratienj
configure not to be ignore
Updates #23: setup intel compiler on macosx
 - fix compilation of ex.h
 - require libc++/clang to compile properly feel++
 - update cln configure script to support intel compiler
Merge branch 'develop' of https://github.com/feelpp/feelpp into develop