Fix builds with CMake configured to falsely return successful detection
when using `check_function_exists()` (and `check_library_exists()`, and
anything based on `try_compile()` that's relying on the linker). After
such mis-detection the build fails when trying to use the feature that
doesn't in fact exist.
The mis-detection is caused by this CMake setting:
```
set(CMAKE_TRY_COMPILE_TARGET_TYPE STATIC_LIBRARY)
```
It is set by default (or on conditions) when using 3rd-party toolchain:
https://github.com/leetal/ios-cmake/blob/master/ios.toolchain.cmake
After this patch the curl build overrides this setting for the duration
of feature tests, and logs a message about it.
Also preset and skip feature tests for known mis-detections:
- `connect()` in `libsocket`
- `getpass_r()`
- `eventfd()` (did not cause an issue due to a separate bug)
- `sendmmsg()` (did not cause an issue because it's Linux-only)
If mis-detections are still seen, the workaround is to force-set the
specific feature by passing `-DHAVE_*=OFF` to cmake.
Also consider passing `-DENABLE_STRICT_TRY_COMPILE=ON` for
`ios.toolchain.cmake` to fix the root cause.
Interestingly curl itself uses this setting to speed up compile-only
detections: be17f298ff#3744
Also:
- OtherTests.cmake: restore original value of
`CMAKE_TRY_COMPILE_TARGET_TYPE`. Before this patch it reset it
to empty.
- OtherTests.cmake: unset a local variable after use, quote a string.
Follow-up to 8e34505776#15164
Follow-up to 8b76a8aeb2#15525
Ref: https://github.com/leetal/ios-cmake/issues/47
Ref: https://gitlab.kitware.com/cmake/cmake/-/issues/18121
Ref: https://cmake.org/cmake/help/latest/variable/CMAKE_TRY_COMPILE_TARGET_TYPE.html
Reported-by: Dan Rosser
Fixes#15557Closes#15559
93e6e4b823