import libtre and use it as a regex.h replacement

on Windows we do not have a regex.h header. libtre is a minimal regex library
with a BSD license which gives us the desired functinality.
Use it for gt_grep().
Windows: make signal functions work
silent clang warning
Merge pull request #131 from gordon/tre

import libtre and use it as a regex.h replacement
Merge pull request #129 from Garonenur/ole_final

Ole Eigenbrod tool