maint: add syntax-check rule to prohibit use of test's -a operator

* cfg.mk (sc_prohibit_test_minus_a): New rule.

cfg.mk

@@ -269,6 +269,12 @@ sc_preprocessor_indentation:
 	  echo '$(ME): skipping test $@: cppi not installed' 1>&2;	\
 	fi
 
+# Using test's -a operator is not portable.
+sc_prohibit_test_minus_a:
+	@re='\<test .+ -[a] '						\
+	msg='use "test C1 && test C2, not "test C1 -''a C2"'		\
+	  $(_prohibit_regexp)
+
 sc_copyright_format:
 	@$(VC_LIST_EXCEPT) | xargs grep -ni 'copyright .*Red 'Hat	\
 	  | grep -v Inc							\