[Polly][CMake] Skip unit-tests in lit if gtest is not available

Summary:
There is a bug in the current lit configurations for the unittests. If gtest is not available, the site-config for the unit tests won't be generated. Because lit recurses through the test directory, the lit configuration for the unit tests will be discovered nevertheless, leading to a fatal error in lit.

This patch semi-gracefully skips the unittests if gtest is not available. As a result, running lit now prints this: `warning: test suite 'Polly-Unit' contained no test`.

If people think that this is too annoying, the alternative would be to pick apart the test directory, so that the lit testsuite discovery will always only find one configuration. In fact, both of these things could be combined. While it's certainly nice that running a single lit command runs all the tests, I suppose people use the `check-polly` make target over lit most of the time, so the difference might not be noticed.

Reviewers: Meinersbur, grosser

Reviewed By: grosser

Subscribers: mgorny, bollu, pollydev, llvm-commits

Tags: #polly

Differential Revision: https://reviews.llvm.org/D34053

llvm-svn: 307651