Install fbjniLibraryConfig.cmake and headers correctly

[jonringer]

It's pretty common for the lib directory to be added
to the cmake modules path, but much less so for
share.

This is to better align with dominate cmake pratices.

Motivation

Why?

Met some awkwardness:

CMake Error at CMakeLists.txt:18 (find_package):
  Could not find a package configuration file provided by "fbjni" with any of
  the following names:

    fbjniConfig.cmake
    fbjni-config.cmake

  Add the installation prefix of "fbjni" to CMAKE_PREFIX_PATH or set
  "fbjni_DIR" to a directory containing one of the above files.  If "fbjni"
  provides a separate development package or SDK, be sure it has been
  installed

Although /nix/store/grkksx8cdka4vmii6mv9fj9handf36m8-fbjni-0.3.0 was on my CMAKE_PREFIX_PATH, but cmake looks for ${CMAKE_PREFIX}/lib/cmake, not ${CMAKE_PREFIX}/share/cmake

Summary

What did you change?

Where it installs the fbjniLibraryConfig.cmake to lib

How does the code work?

GNUInstallDirs + CMake

Why did you choose this approach?

Most common pattern that I've seen

Test Plan

How did you test this change?

mkdir build && cd build && mkdir tmp_install  && cmake .. -DCMAKE_INSTALL_PREFIX=$PWD/tmp_install && make install

Adding a downstream project to test further would probably be good

Any change that adds functionality should add a unit test as well.

[jonringer]

another option would be to install in both locations, to avoid breaking changes for individuals

[jonringer]

Also, added the headers as part of the install target, so that cmake --build --target install should be all that downstream users need to do for installation.