fix tau installation issue  (#2269)
fix tau installation issue  (#2269)

* fix tau installation issue : setup_environment() is
called before install phase when 'Makefile.*' doesn't
exist (causing list index out of range error).

* Added detailed comment suggested by @alalazo

matthiasdiener/spack,iulian787/spack,TheTimmy/spack,mfherbst/spack,iulian787/spack,TheTimmy/spack,skosukhin/spack,matthiasdiener/spack,tmerrick1/spack,tmerrick1/spack,tmerrick1/spack,mfherbst/spack,EmreAtes/spack,LLNL/spack,lgarren/spack,matthiasdiener/spack,mfherbst/spack,iulian787/spack,EmreAtes/spack,iulian787/spack,EmreAtes/spack,LLNL/spack,skosukhin/spack,lgarren/spack,krafczyk/spack,mfherbst/spack,krafczyk/spack,iulian787/spack,EmreAtes/spack,LLNL/spack,krafczyk/spack,tmerrick1/spack,skosukhin/spack,lgarren/spack,krafczyk/spack,LLNL/spack,matthiasdiener/spack,krafczyk/spack,TheTimmy/spack,lgarren/spack,LLNL/spack,lgarren/spack,tmerrick1/spack,skosukhin/spack,skosukhin/spack,TheTimmy/spack,matthiasdiener/spack,mfherbst/spack,EmreAtes/spack,TheTimmy/spack