The oldest cmake supported by curl is v3.7.0, which already has such
guard (using `PKG_CONFIG_EXECUTABLE`) inside `pkg_check_modules()`. The
advantage of leaving that guard to CMake is that it will define/reset
all output variables, while the manual guard doesn't do this and also
leaves for example `NETTLE_FOUND` undefined.
Delete the single use of this guard from the recently added `nettle`
detection, where I included it by accident. Then possibly re-introduce
it universally if we find it useful after more evaluation.
Follow-up to 669ce42275#14285Closes#14309
`nettle` is a direct dependency of curl, when building with GnuTLS.
Add a new `Find` module to detect it.
Also:
- GHA/macos: drop `nettle` hack no longer necessary.
- add `nettle` to `libcurl.pc`.
- also add `nettle` to `libcurl.pc` in autotools builds.
Follow-up to 781242ffa4#11967Closes#14285
