mmg : add variant to install private headers for parmmg packaged

[prudhomm]

add private variant to install private headers thanks to MMG_INSTALL_PRIVATE_HEADERS needed by parmmg

/cc @jcortial-safran

• closes feelpp/spack#14

[jcortial-safran]

I would prefer a more explicit variant name, e.g. private_headers.

The variant should be also appear as a dependency in the parmmg package : https://github.com/spack/spack/blob/develop/var/spack/repos/builtin/packages/parmmg/package.py

[jcortial-safran]

Unless I am mistaken the option is only available for version 5.7.0 or later : https://github.com/MmgTools/mmg/commit/9f98ef4050a59b56e651e1641e07b2b1253917c0

So the option should not be available for versions earlier than 5.7.0.

[jcortial-safran]

LGTM. @jcortial-safran ?

I have suggested a couple of minor changes.

[tldahlgren]

Ping me with a comment here when this is ready to merge.

[prudhomm]

@tldahlgren @jcortial-safran I believe I applied the required changes.

[jcortial-safran]

LGTM @tldahlgren