Add support for pkg-config

[watersofoblivion]

Summary

Adds support for pkg-config.

Details

Generates and installs cucumber-cpp.pc and cucumber-cpp-nomain.pc files. The version number is hard-coded at 0.6 (one minor past the last tagged version) in the .pc files since there doesn't appear to be a version variable to use.

The implementation was largely lifted from Googletest.

Motivation and Context

This was added to ease integration with other build systems (specifically, to be usable by flatpak + meson.)

How Has This Been Tested?

As part of the build there are no unit tests, but the change was tested manually.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue).
• [x] New feature (non-breaking change which adds functionality).
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected).

Checklist:

• [x] It is my own work, its copyright is implicitly assigned to the project and no substantial part of it has been copied from other sources (including Stack Overflow). In rare occasions this is acceptable, like in CMake modules where the original copyright information should be kept.
• [x] I'm using the same code standards as the existing code (indentation, spacing, variable naming, ...).
• [ ] I've added tests for my code.
• [x] I have verified whether my change requires changes to the documentation
• [x] My change either requires no documentation change or I've updated the documentation accordingly.
• [x] My branch has been rebased to master, keeping only relevant commits.

[coveralls]

Coverage remained the same at 62.47% when pulling 1212a2af0fda497754db7dfe36ffd3e176411d4a on watersofoblivion:add-pkg-config into dd424c1a900cb41c5db9d481714eb3471ce195e3 on cucumber:master.