configure.ac: use AS_IF instead of if/then/[else/]fi
configure.ac: use AS_IF instead of if/then/[else/]fi

AS_IF is always safe, whereas if/fi usually works in practice, but
sometimes fails to do the right thing with Autoconf macro dependencies.

See <https://bugzilla.gnome.org/show_bug.cgi?id=681413> for more
information.

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