configure.ac: use AS_CASE instead of case/esac
configure.ac: use AS_CASE instead of case/esac

This is the same as AS_IF, but for "case". It's safer for the same
reasons.

Signed-off-by: Simon McVittie <2b470b34502dbf3ba4bcc04ea7e64f4e0e104f33@collabora.co.uk>
Reviewed-by: Jonny Lamb <3d1fb3642aaab2ff70e3392a5ab916bf99eed6a3@collabora.co.uk>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=53445

Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib