Make conf-autoconf a dev dependency

[palmskog]

The current released version of validsdp, coq-validsdp.0.7, has conf-autoconf as a build dependency:

"conf-autoconf" {build}

However, it can be an inconvenience to users of the released opam repo to have to install autoconf (a system package), just to install a Coq package. The current best practice is therefore to declare conf-autoconf a development build dependency:

"conf-autoconf" {dev & build}

This is because released packages are expected to be based on tarballs generated by autoconf, which removes the need for autoconf itself. See Flocq for examples of this, e.g., the package coq-flocq.3.4.3.

[proux01]

@palmskog thanks, did that for the OPAM files in this repo and coq/opam#2077 completely removes the conf-autoconf dependencies as the configure.ac source files of autoconf are not even shipped in the tarballs.