ftp/curl: avoid auto-enabling ZSTD, make explicit option; ensure GSSAPI compilation.
Since August 2020, curl would auto-enable ZSTD if available while
configuring, this would however not propagate to the LIB_DEPENDS.
Make sure to explicitly control ZSTD by introducing an option that
defaults to off.
While here, also mark BROKEN if GSSAPI_BASE is selected while
krb5 is installed when building.
PR: 250414
Approved by: sunpoet@ (maintainer timeout, 14d)