build: make gyp user defined variables lowercase

I mistakenly introduced user defined variables using uppercase
characters, reading the gyp documentation they state:
"Predefined variables. By convention, these are named with
CAPITAL_LETTERS. Predefined variables are set automatically by GYP"
and also "By convention, user-defined variables are named with
lowercase_letters."

This commit renames the user defined variables to lowercase to follow
the above mentioned convention.

PR-URL: https://github.com/nodejs/node/pull/16238


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>