With GnuTLS 3.8.0+ the build-time SRP feature detection always succeeds.
It's also disabled by default in these GnuTLS versions.
When using TLS-SRP without it being available in GnuTLS, report
the correct error code `CURLE_NOT_BUILT_IN`, replacing the out of memory
error reported before this patch.
Also add comments to autotools and cmake scripts about this feature
detection property.
Detecting it at build-time would need to run code which doesn't work
in cross-builds. Once curl requires 3.8.0 as minimum, the build-time
checks can be deleted.
```
# before:
curl: (27) gnutls_srp_allocate_client_cred() failed: An unimplemented or disabled feature has been requested.
# after:
curl: (4) GnuTLS: TLS-SRP support not built in: An unimplemented or disabled feature has been requested.
```
Ref: dab063fca2
Ref: a21e89edacCloses#19365
8e6149598b