Makefile: Implement clean target

Change-Id: Ia21b0f184bfd454b3835774949f581a27860a80a
diff --git a/Makefile b/Makefile
index ee6300e..c36aa9a 100644
--- a/Makefile
+++ b/Makefile
@@ -15,4 +15,10 @@
 	$(MAKE) -C selftest check
 	@echo "make check: success"
 
+clean:
+	$(MAKE) -C selftest clean
+	$(MAKE) -C doc/manuals clean
+	@find . -name "*__pycache__" -type d -print0 | xargs -0 rm -rvf
+	@rm -fv ./src/osmo_gsm_tester/_version.py
+	@rm -fv ./version
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