Makefile: use PYTHON variable instead of python3 (#1101)

* Makefile: use PYTHON variable instead of python3

* Makefile: make Python call silent

Co-Authored-By: Antoine <43954001+awecx@users.noreply.github.com>

Author: rffontenelle

Makefile

@@ -99,7 +99,7 @@ serve:
 
 .PHONY: progress
 progress:
-	@python3 -c 'import sys; print("{:.1%}".format(int(sys.argv[1]) / int(sys.argv[2])))'  \
+	@$(PYTHON) -c 'import sys; print("{:.1%}".format(int(sys.argv[1]) / int(sys.argv[2])))'  \
 	$(shell msgcat *.po */*.po | msgattrib --translated | grep -c '^msgid') \
 	$(shell msgcat *.po */*.po | grep -c '^msgid')