naming_scheme: '${PACKAGE}/${PACKAGE}-${VERSION}-${OPTIONS}-${COMPILERNAME}-${COMPILERVER}'

[vagrant@sandbox ~]$ module avail emacs

-------------- /vagrant/spack/share/spack/modules/linux-centos7-x86_64 --------------
emacs/emacs-25.1?toolkit=gtk?-gcc-4.8.5
[vagrant@sandbox ~]$ cd /vagrant/spack/share/spack/modules/linux-centos7-x86_64/emacs/
[vagrant@sandbox emacs]$ ls -l
total 4
-rw-r--r-- 1 vagrant vagrant 974 Jan 23 10:22 emacs-25.1 toolkit=gtk -gcc-4.8.5

        toolkit = spec.variants['toolkit'].value
        if '+X' in spec:
            if toolkit not in ('gtk', 'athena'):
                raise InstallError("toolkit must be in (gtk, athena), not %s" %
                                   toolkit)
            args = [
                '--with-x',
                '--with-x-toolkit={0}'.format(toolkit)
            ]
        else:
            args = ['--without-x']