build-sys: use AS_IF instead of shell if clause

Shell if clauses can cause problems.

Change-Id: Ia5fddc78ff43517c37d5abd79f07b35fd18a9058
Reference: http://www.gnu.org/software/autoconf/manual/autoconf-2.67/html_node/Limitations-of-Builtins.html#if
Reviewed-on: http://gerrit.openafs.org/10009
Reviewed-by: Simon Wilkinson <simonxwilkinson@gmail.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>