Changes in FindPetsc and related modules from Flow123d

First, this is my first serious use of git, github and pull request. Sorry for any inconveniences.

1) Changes in support modules only fix bugs found in our configuration on cygwin. I hope not to broke it elsewhere. Namely I can not test it on windows outside of cygwin.
2) In FindPetsc there are some fixes, better error messages, ability to export more variables from PETSc makefiles, and only warn if we can compile but can not run test source (practical on systems with PBS that disallow mpirun on frontends)
3) InstallPetsc - it works for us, but may need some more work for general usage; I have turn fixed URL to a variable