Add --with-libgssapi_krb5 to match libsodium and pgm.
Add support for --with/--without libgssapi_krb5
Pass --with/--without-libgssapi_krb5 to configure.
Correct libgssapi_krb5 packages.
Support both --enable-eventfd and --disable-eventfd options.
Merge pull request #5 from johntconklin/master

backport configure and rpmbuild fixes.