Silence BUILD_SHARED_LIBS=ON gtest warnings
Silence BUILD_SHARED_LIBS=ON gtest warnings

When `BUILD_SHARED_LIBS=ON`, gtest emits a couple of cmake warnings, one
about MACOSX_RPATH and one about a missing link_directory. We can quiet
both warnings by setting these properties.

We could also just change gtest's CMakeLists.txt in our vendor
directory, but I'm submitting this change on the principle that the
vendor subdirectories should stay pristine. If vendored software can be
tweaked in our tree, then we should discard this patch and instead fix
things in `vendor/gtest-1.7.0/CMakeLists.txt`.

lucadelu/PDAL,lucadelu/PDAL,lucadelu/PDAL,lucadelu/PDAL