use AS_IF, AS_CASE instead of if/fi and case/esac

Submitted by Simon McVittie

Assigned to Telepathy bugs list

Link to original bug (#54115)

Description

+++ This bug was initially created as a clone of Bug #53445 +++

Ordinary shell conditionals with "if" or "case" aren't actually safe in configure.ac; it's sometimes necessary to use AS_IF, AS_CASE. Ideally, we should do that consistently across all projects.