Support TIMING environment variable
When using coq_makefile, if TIMING is nonnull -time will be passed to coqc and output redirected to a file. This allows for instance integration into the Jenkins benchmarking to produce per-line timing differences, see eg https://ci.inria.fr/coq/job/benchmark-part-of-the-branch/ws/547/html/coq-hott/theories/Categories/Grothendieck/ToSet/Morphisms.v.html (attached for when it gets garbage collected and for those you don't have jenkins access: Morphisms.v.html).
Relevant parts of the coq_makefile implementation: https://github.com/coq/coq/blob/15998894ff76b1fa9354085ea0bddae4f8f23ddf/tools/CoqMakefile.in#L209-L210 https://github.com/coq/coq/blob/15998894ff76b1fa9354085ea0bddae4f8f23ddf/tools/CoqMakefile.in#L655-L663
I tried to do it in your remakefile.in but I don't know how to do the conditional redirection in remake without something like if [ $TIMING ]; @COQC@ -time args > bla; else @COQC@ args; fi
so I figured I would let the experts do it.