Build: don't write __pycache__ files into lib folder for make commands

These scripts are not performance critical, and the cache files could
potentially interfere with library updates in the future.

GNUmakefile

@@ -236,6 +236,10 @@ ifndef PYTHON
 		ifeq (, $(shell command -v $(PYTHON)))
 			PYTHON:=python
 		endif
+	else
+		# Don't generate __pycache__ files in lib folder, they
+		# can interfere with updates.
+		PYTHON:=$(PYTHON) -B
 	endif
 endif