configure: Replace shell constructs with M4sh macros

AS_IF and AS_CASE should be used instead of if and case to properly
handle macros in conditional branches.

Also guard all tests against empty variables and embedded spaces which
makes it possible to remove some variable defaults.

Change-Id: Ib675177690d406a80ee98dd5d105296a7cdc7b8a
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/3669


Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>