The new detection method also allows to enable librtmp without using
OpenSSL as a curl TLS backend at the same time.
Also:
- implement manual version detection for librtmp.
Version info is in hex. With CMake 3.13 and newer, extract it as a hex
number. With earlier CMake version, just strip the leading zeroes.
Doing more here seems overkill because librtmp has been standing
at 2.3/2.4 for a decade now. Bumping into hex digits seems unlikely
before deprecating CMake 3.13 support.
librtmp advertises v2.4 via its `pkg-config` module, and v2.3 via
its public header. The latter shows up in `curl -V` and either can
be shown at configure-time depending on detection method.
This isn't a curl bug.
- GHA/macos: enable rtmp in a job.
- apply the "half-detection" fix to the Find module.
`librtmp` is also affected (in CI too), because it depends on libssl and
libcrypto.
Closes#15832
421e592db2