cmake: determine when to use port.h

[sergiud]

Allow building using MSYS2 (without MinGW).

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (fe46778) 59.42% compared to head (da3b8e7) 59.42%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1037 +/- ## ======================================= Coverage 59.42% 59.42% ======================================= Files 19 19 Lines 2541 2541 Branches 908 908 ======================================= Hits 1510 1510 Misses 686 686 Partials 345 345 ``` | [Files](https://app.codecov.io/gh/google/glog/pull/1037?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=google) | Coverage Δ | | |---|---|---| | [src/stacktrace\_windows-inl.h](https://app.codecov.io/gh/google/glog/pull/1037?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=google#diff-c3JjL3N0YWNrdHJhY2Vfd2luZG93cy1pbmwuaA==) | `66.66% <ø> (ø)` | | | [src/utilities.h](https://app.codecov.io/gh/google/glog/pull/1037?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=google#diff-c3JjL3V0aWxpdGllcy5o) | `100.00% <ø> (ø)` | |