Make the 'gmake me' work even when the command-line name for the make utility is gmake.
Makefile -- call build/exportmt.pl with the MAKE environment variable set to the execution path of the make utility being used.
build/Build.pm -- instead of hardcoding 'make,' refer to $ENV{MAKE}.

Make it work when exportmt.pl is executed from command line.
Use 'make' when $ENV{MAKE} is undefined.