fix: cleanup USE_FB2 macros.

[romange]

1. Move uring source files under util/fibers.
2. Now third_party package builds with the same cxx compiler.

[codecov-commenter]

Codecov Report

Patch coverage: 67.44% and no project coverage change.

Comparison is base (debfbef) 76.47% compared to head (1b503d5) 76.47%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #138 +/- ## ======================================= Coverage 76.47% 76.47% ======================================= Files 98 98 Lines 7106 7106 ======================================= Hits 5434 5434 Misses 1672 1672 ``` | [Files Changed](https://app.codecov.io/gh/romange/helio/pull/138?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Roman+Gershman) | Coverage Δ | | |---|---|---| | [util/fibers/listener\_interface.cc](https://app.codecov.io/gh/romange/helio/pull/138?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Roman+Gershman#diff-dXRpbC9maWJlcnMvbGlzdGVuZXJfaW50ZXJmYWNlLmNj) | `70.00% <ø> (ø)` | | | [util/fibers/uring\_proactor.cc](https://app.codecov.io/gh/romange/helio/pull/138?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Roman+Gershman#diff-dXRpbC9maWJlcnMvdXJpbmdfcHJvYWN0b3IuY2M=) | `53.91% <ø> (ø)` | | | [util/fibers/uring\_proactor.h](https://app.codecov.io/gh/romange/helio/pull/138?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Roman+Gershman#diff-dXRpbC9maWJlcnMvdXJpbmdfcHJvYWN0b3IuaA==) | `90.00% <ø> (ø)` | | | [util/fibers/uring\_socket.cc](https://app.codecov.io/gh/romange/helio/pull/138?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Roman+Gershman#diff-dXRpbC9maWJlcnMvdXJpbmdfc29ja2V0LmNj) | `75.33% <ø> (ø)` | | | [util/fibers/submit\_entry.h](https://app.codecov.io/gh/romange/helio/pull/138?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Roman+Gershman#diff-dXRpbC9maWJlcnMvc3VibWl0X2VudHJ5Lmg=) | `64.10% <64.10%> (ø)` | | | [util/fibers/uring\_socket.h](https://app.codecov.io/gh/romange/helio/pull/138?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Roman+Gershman#diff-dXRpbC9maWJlcnMvdXJpbmdfc29ja2V0Lmg=) | `100.00% <100.00%> (ø)` | |

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