Allow usage of system provided status-code

[BurningEnlightenment]

Purpose

The current cmake buildsystem vendors the status-code headers which is at odds with most package managers which require using the version provided through them. Furthermore it is unergonomic for users who need to include additional categories like com_code.hpp.

Solution Sketch

Add a cmake feature toggle which instructs cmake to look for an installed status-code package. If configured that way, we would not install any status-code headers but reference the imported status-code::hl target.

Additional explanatory comments

• I upgraded the minimum required cmake version to 3.5 as that is already required by quickcpplib. I think it should be considered to require an even newer version as a similar discussion on llvm-dev last year revealed that all current LTS platforms support at least 3.6.1 (or 3.13.4 if one is willing to sacrifice RHEL 6 and Debian 9 support).
• The changes should be backward compatible and consumers have to opt in.

[github-actions]

Unit Test Results

  1 files   -     2  38 suites   - 144   0s ⏱️ ±0s
40 tests  -     1  40 ✔️  -     1  0 💤 ±0  0 ❌ ±0 
40 runs   - 152  40 ✔️  - 152  0 💤 ±0  0 ❌ ±0 

Results for commit 147ec1e. ± Comparison against base commit 99d1aac.