Introduce a make recipe for flake8

Add a make recipe to enable developers
to easily run flake8 if it is available.
This will help enforce good python coding
standards.

Signed-off-by: Giuseppe Di Natale <dinatale2@llnl.gov>

Makefile.am

@@ -39,7 +39,7 @@ dist-hook:
 	sed -i 's/Release:[[:print:]]*/Release:      $(RELEASE)/' \
 		$(distdir)/META
 
-checkstyle: cstyle shellcheck
+checkstyle: cstyle shellcheck flake8
 
 cstyle:
 	@find ${top_srcdir} -name '*.[hc]' ! -name 'zfs_config.*' \
@@ -62,6 +62,11 @@ cppcheck:
 		cppcheck --quiet --force --error-exitcode=2 ${top_srcdir}; \
 	fi
 
+flake8:
+	@if type flake8 > /dev/null 2>&1; then \
+		flake8 ${top_srcdir}; \
+	fi
+
 ctags:
 	$(RM) tags
 	find $(top_srcdir) -name .git -prune -o -name '*.[hc]' | xargs ctags