Generate and install a pkg-config file

[acowley]

This is a

• Breaking change
• New feature
• Bugfix

I have

• Merged in the latest upstream changes
• Updated CHANGELOG.md
• Updated README.md
• Run the tests

[acowley]

I ran into a need for this while working with easyloggingpp using the nix package manager. I currently generate the .pc file in a nix expression, as pkg-config and cmake work quite nicely together in the nix world, but it would be better if the cmake files of nix users could work outside of the nix environment.

I have not yet updated the CHANGELOG, but would be happy to do so if this new feature is acceptable.

[abumq]

looks good, except for the branch, can you please push to develop branch instead? also you can update CHANGELOG if you like.

[acowley]

I've rebased onto develop and updated CHANGELOG.

[acowley]

Does this look okay to merge, @abumusamq?

[acowley]

Another ping to see if we can get this merged.

[acowley]

Any movement on this?

[abumq]

Thanks for this PR @acowley
I have made manual updates (c1b1285). This should be up in next release