* Make tests buildable independently
This makes "tests" buildable as a separate project that uses
find_package(pybind11 CONFIG) when invoked independently.
This also moves the WERROR option into tests/CMakeLists.txt, as that's
the only place it is used.
* Use Eigen 3.3.1's cmake target, if available
This changes the eigen finding code to attempt to use Eigen's
system-installed Eigen3Config first. In Eigen 3.3.1, it exports a cmake
Eigen3::Eigen target to get dependencies from (rather than setting the
include path directly).
If it fails, we fall back to the trying to load allowing modules (i.e.
allowing our tools/FindEigen3.cmake). If we either fallback, or the
eigen version is older than 3.3.1 (or , we still set the include
directory manually; otherwise, for CONFIG + new Eigen, we get it via
the target.
This is also needed to allow 'tests' to be built independently, when
the find_package(Eigen3) is going to find via the system-installed
Eigen3Config.cmake.
* Add a install-then-build test, using clang on linux
This tests that `make install` to the actual system, followed by a build
of the tests (without the main pybind11 repository available) works as
expected.
To also expand the testing variety a bit, it also builds using
clang-3.9 instead of gcc.
* Don't try loading Eigen3Config in cmake < 3.0
It could FATAL_ERROR as the newer cmake includes a cmake 3.0 required
line.
If doing an independent, out-of-tree "tests" build, the regular
find_package(Eigen3) is likely to fail with the same error, but I think
we can just let that be: if you want a recent Eigen with proper cmake
loading support *and* want to do an independent tests build, you'll
need at least cmake 3.0.
