Simplify the selection for the dynamic thread pool implementation

[BurningEnlightenment]

• If on window use native OS APIs (libdispatch isn't available anyways).
• If on linux use the superior custom implementation unless the user explicitly requests libdispatch (which is incompatible with gcc).
• Otherwise require libdispatch.

Note that I opted to use the custom implementation on linux by default even if libdispatch is available in order to prevent ODR violations caused mixing of gcc and clang.

Fixes #94

[github-actions[bot]]

Unit Test Results

2 files   -     4  0 suites   - 192   0s :stopwatch: - 34m 26s 0 tests  -   57  0 :heavy_check_mark:  -   57  0 :zzz: ±0  0 :x: ±0  0 runs   - 360  0 :heavy_check_mark:  - 360  0 :zzz: ±0  0 :x: ±0 

Results for commit 7986b922. ± Comparison against base commit b1825a23.

[github-actions[bot]]

Unit Test Results

    6 files  ±0  192 suites  ±0   25m 15s :stopwatch: - 9m 11s   57 tests ±0    57 :heavy_check_mark: ±0  0 :zzz: ±0  0 :x: ±0  360 runs  ±0  360 :heavy_check_mark: ±0  0 :zzz: ±0  0 :x: ±0 

Results for commit 7986b922. ± Comparison against base commit b1825a23.

[github-actions[bot]]

Unit Test Results

    4 files   -     2    62 suites   - 130   31m 51s :stopwatch: - 2m 35s   58 tests +    1    57 :heavy_check_mark: ±    0  0 :zzz: ±0  1 :x: +1  120 runs   - 240  119 :heavy_check_mark:  - 241  0 :zzz: ±0  1 :x: +1 

For more details on these failures, see this check.

Results for commit 7986b922. ± Comparison against base commit b1825a23.

[github-actions[bot]]

Unit Test Results

2 files   -     4  0 suites   - 192   0s :stopwatch: - 34m 26s 0 tests  -   57  0 :heavy_check_mark:  -   57  0 :zzz: ±0  0 :x: ±0  0 runs   - 360  0 :heavy_check_mark:  - 360  0 :zzz: ±0  0 :x: ±0 

Results for commit 7986b922. ± Comparison against base commit b1825a23.

[github-actions[bot]]

Unit Test Results

    4 files   -     2    62 suites   - 130   14m 31s :stopwatch: - 19m 55s   58 tests +    1    58 :heavy_check_mark: +    1  0 :zzz: ±0  0 :x: ±0  120 runs   - 240  120 :heavy_check_mark:  - 240  0 :zzz: ±0  0 :x: ±0 

Results for commit 7986b922. ± Comparison against base commit b1825a23.

[github-actions[bot]]

Unit Test Results

    6 files  ±0  192 suites  ±0   35m 55s :stopwatch: + 1m 29s   57 tests ±0    57 :heavy_check_mark: ±0  0 :zzz: ±0  0 :x: ±0  360 runs  ±0  360 :heavy_check_mark: ±0  0 :zzz: ±0  0 :x: ±0 

Results for commit 7986b922. ± Comparison against base commit b1825a23.

[ned14]

Broken on MacOS? https://github.com/ned14/llfio/runs/8193009872?check_suite_focus=true