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.

Change-Id: Iaf14a82134cc2dcf3c23b1e5a0ed65606e2487bb
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>