Fixes status message when PETSc couldn't be found.
Removes broken conditional check.

The FindPetsc module would check for a PETSC_ROOT and not attempt to look for
mpi and the petsc header if this was not set, which it in most cases won't be.
If the variable is set then the conditional include is redundant, as it will
search the explicitly passed paths first anyways.
FindPets searches default paths w/ general namings

Removed the NO_DEFAULT_PATH flag as this really isn't working for most systems
(where PETSc will be installed from a repository and in /usr or /). Path suffix
parameters extended to accept more version-less configs too, to be compatible
with a default PETSc (debian style) install.
FindPetsc uses system-provided MPI if available

When building with USE_MPI=ON the FindPetsc now uses the system provided
MPI if available. Falls back to looking for the PETSc provided unimpi for
sequential builds.
PETSC_ROOT now sets NO_DEFAULT_PATH

Allows a specific PETSC_ROOT to override and replace default search paths. Now,
if PETSC_ROOT is passed and the find_library calls cannot find the libraries,
the makefile will only search in user provided directories.
Simplifcations in FindPetsc logic

Removes a lot of the conditionals in the makefile. As reflected in the
comments, find_* won't run if the target variable is already set, and appending
empty list elements changes nothing. In order to make reading the file easier
these conditionals have mostly been removed.
Merge pull request #1 from jorgekva/linsolver_petsc

Improvements to FindPetsc.cmake module