Fix paths for Ubuntu 16.04+ and Debian 9

[mdave]

This is another approach that I've adopted to fix issue #16, but different in style to pull request #17 so might serve as an alternative choice. Gotta say these packages are a bit of a pain to use from a CMake perspective, but this is the least intrusive solution I could come up with.

The issue is that newer Ubuntu and Debian packages construct libraries that install into locations of the form: /usr/lib/petscdir/VERSION/ARCH/lib/libpetsc_ARCH.so. Following the usual Debian guidelines, these (and header includes) are then symlinked into places such as /usr/lib/libpetsc.so and friends, depending on the real/complex architecture choice selected in update-alternatives. This means that defining PETSC_DIR and PETSC_ARCH no longer works on these distributions, since the CMake configuration cannot find the libpetsc_ARCH.so library.

The solution I have used here is to instead direct CMake to look inside /usr/lib/petsc first, which in these newer distributions is a symlink to the petscdir/VERSION/ARCH that has been selected by the user in update-alternatives. This has the advantage of no longer needing to look in hard-coded version-specific locations or hack around with PETSC_ARCH, and from the command line the library is detected automatically without manually specifying paths. However, the disadvantage is that we now need to additionally look for libpetsc_real.so and libpetsc_complex.so. Right now no other distributions use these alternative names, as far as I know, but I guess this could potentially be a cause for concern longer-term.

[mdave]

And just to clarify: when I say the packages are a pain, I mean the Debian distribution and not your CMake configuration 😄