[ 13138]
perl.c change to use HAS_PROCSELFEXE, also
tweak to $^X test to comprehend full path to real executable
being returned (like Cygwin as it happens...)
p4raw-link: @13138 on //depot/perlio:
04fee9b5a11624a5e33d02ff7b7129631d312bd9
p4raw-id: //depot/perl@13140