add ebuild support

[goatshriek]

Adds an ebuild template that is generated at configuration time and can be used to install the library on Gentoo systems.

New build targets and configuration options are also included in this change to improve installation support. These include options to install manpages and examples and a build-test target to compile functionality tests without running them.

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (86c9296) 91.55% compared to head (8aa5190) 91.55%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## latest #355 +/- ## ======================================= Coverage 91.55% 91.55% ======================================= Files 43 43 Lines 3660 3660 Branches 474 474 ======================================= Hits 3351 3351 Misses 216 216 Partials 93 93 ```

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[sonarcloud[bot]]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
No Duplication information

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