if camlp5 binary is in path, ask itself where it is

Patch by Thomas Braibant

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/branches/v8.4@17040 85f007b7-540e-0410-9357-904b9bb8a0f7
Allow the use of make 4.00

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/branches/v8.4@17041 85f007b7-540e-0410-9357-904b9bb8a0f7