build: Check for Guile-SQLite3.

* m4/guix.m4 (GUIX_CHECK_GUILE_SQLITE3): New macro.
* configure.ac: Use it and define 'HAVE_GUILE_SQLITE3'.
* guix/self.scm (specification->package): Add "guile-sqlite3".
(compiled-guix)[guile-sqlite3]: New variable.
[dependencies]: Add it.