openssh: enable authentication via Kerberos through GSSAPI (#31086)

Add variant +gssapi to enable authentication via Kerberos through GSSAPI

When openssh is installed at sites using Kerberos, openssh needs to auth
via Kerberos through GSSAPI in order to work in such environments.

var/spack/repos/builtin/packages/openssh/package.py

@@ -45,6 +45,9 @@ class Openssh(AutotoolsPackage):
     version('6.7p1', sha256='b2f8394eae858dabbdef7dac10b99aec00c95462753e80342e530bbb6f725507')
     version('6.6p1', sha256='48c1f0664b4534875038004cc4f3555b8329c2a81c1df48db5c517800de203bb')
 
+    variant('gssapi', default=True, description='Enable authentication via Kerberos through GSSAPI')
+
+    depends_on('krb5', when='+gssapi')
     depends_on('openssl@:1.0', when='@:7.7p1')
     depends_on('openssl')
     depends_on('libedit')
@@ -67,6 +70,8 @@ def configure_args(self):
         # least newer versions want to create the directory during the
         # install step and fail if they cannot do so.
         args = ['--with-privsep-path={0}'.format(self.prefix.var.empty)]
+        if self.spec.satisfies('+gssapi'):
+            args.append('--with-kerberos5=' + self.spec['krb5'].prefix)
 
         # Somehow creating pie executables fails with nvhpc, not with gcc.
         if '%nvhpc' in self.spec: