Windows: permit perl command to be explicitly set

Windows: permit perl command to be explicitly set

Add PERL variable to the build system.  If not specified
externally the variable will be set to 'perl'.  However,
ActiveState Perl should be used and not Cygwin Perl.  The build
environment should indicate that by specifying a PERL setting.

SET PERL=c:\perl64\bin\perl.exe

or similar.

Reviewed-on: http://gerrit.openafs.org/5115
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
(cherry picked from 4138a778fe821493c726826ffc4715f5f46222d9)

Change-Id: Ifd5ba10176ba1a7d6fda7506a0a4c250ad1945f6
Reviewed-on: http://gerrit.openafs.org/5143
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>