use AS_IF, AS_CASE instead of if/fi and case/esac
Submitted by Simon McVittie
Assigned to Xavier Claessens @xclaesse
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.