tests: loosen/tighten the always_defined_macros check

* cfg.mk (.re-defmac): Generate better regexps: allow white space
before the '#', and append a word-boundary requirement.
Without the latter, #define NULL_DEV ... would evoke a false-positive.

cfg.mk

@@ -128,7 +128,7 @@ gl_extract_significant_defines_ = \
 	  done;								\
 	) | sort -u							\
 	  | grep -Ev '^ATTRIBUTE_NORETURN'				\
-	  | sed 's/^/^# *define /'					\
+	  | sed 's/^/^ *# *define /;s/$$/\\>/'				\
 	  > $@-t
 	@mv $@-t $@