make install installs Google test libs but not NativeJIT

[joto]

make install installs gtest and gmock libs and includes, which should probably not be installed, but it doesn't install NativeJIT and CodeGen libs and includes.

[danluu]

Good catch! We obviously don't include NativeJIT this way so the it just happens to do this by coincidence since we weren't thinking about this when we put together the CMakeLists.txt.

If no one else knows the right way to do this, we'll take a look at fixing up our CMakeLists to do the right thing. Sorry about this! We're relatively new to using cmake.

Note to self: we may want to do something like https://github.com/glfw/glfw/issues/30. Or not.