Avoid target conflicts with glslang and googletest.

Avoid target conflicts with glslang and googletest.

This makes it easier to include spirv-tools into larger cmake-based
projects, which may already include glslang or googletest. It is
currently difficult to do this because of target clashes and a
hardcoded googletest path.

glslang defines a target named SPIRV, so rename ours to SPIRV-TOOLS.

A googletest subdirectory may already be added somewhere else, so if
the external/googletest directory does not exist, probe whether there
is a gtest target defined already. This makes spirv-tools work
out-of-the-box when plopped into a larger project already containing
googletest; otherwise the README.md procedure still works as before.