install missing `conf-*` packages

[gasche]

The following conf-* packages are currently not installed, which makes software which depends on them unbuildable under opam-builder:

• conf-libuv (libuv1-dev)
• conf-lua (liblua5.2-dev")
• conf-pic-switch (relies on $opam_root'/'$ocaml_version'+PIC' being available)

[lefessan]

• conf-lua depexts are installed, but it fails to detect them
• conf-libuv is now installed
• conf-pic-switch: when I have more time...