Export CMake targets about scr

[adammoody]

Thanks @white238 . I'm hoping we can look at this for the next release.

Related: #319

[adammoody]

Thanks again for your suggestion, @white238 . Things have improved on this front.

First, one may build SCR from a release tarball. The release tarball is packaged with the source for SCR and many of its component libraries (kvtree, spath, rankstr, redset, shuffile, er, lwgrp, and dtcmp). It compiles all source into a single libscr library. This saves one from having to download and build each component library individually. It's still possible to build each dependency separately, but it's not required.

These tarballs are attached as assets on the Releases page, e.g.,
https://github.com/LLNL/scr/releases/download/v3.0.1/scr-v3.0.1.tgz

Steps to build a release tarball:
https://scr.readthedocs.io/en/v3.0/users/build.html#cmake

Second, we've added an scrConfig.cmake that defines targets including scr::scr and scr::scr-static. These targets add the compile and link flags for SCR and its dependencies. There is a corresponding scrConfig.cmake for both the release tarball and the separate-component build. We still need to cut a new SCR release for the CMake config file.