configure: portability fixes - take 2

It is useful to sometime run on a strictly POSIX-compliant
shell... :-/

And it is sometime interesting to refresh Q-patches before
finishing the series... :-(

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 # m68k specific configuration file

 ## depends on EXPERIMENTAL

 ##

 ## select ARCH_SUPPORTS_32

 ## select ARCH_DEFAULT_32

 ## select ARCH_DEFAULT_BE

 ## select ARCH_SUPPORT_CPU

 ##

 ## help The m68k architecture