Syntax check rules broken if 'exclude' is not defined

[Daniel P. Berrange]

I noticed that many (most) of the libvirt syntax-check rules were not
generating any results, despite me introducing obvious violations.

>From my debugging the problem appears to be caused by this commit:

  commit 727075d03c670affa68f71313979781f5ba9bbfc
  Author: Eric Blake <address@hidden>
  Date:   Thu Mar 1 14:51:31 2012 -0700

    maint.mk: add per-line exclusions to prohibitions


If I revert the following hunk from maint.mk


@@ -258,6 +268,7 @@ define _sc_search_regexp
    if test -n "$$files"; then                                          \
      if test -n "$$prohibit"; then                                     \
        grep $$with_grep_options $(_ignore_case) -nE "$$prohibit" $$files \
+         | grep -vE "$${exclude-^$$}"                                  \
          && { msg="$$halt" $(_sc_say_and_exit) } || :;                 \
      else                                                              \
        grep $$with_grep_options $(_ignore_case) -LE "$$require" $$files \


then the syntax-check rules work again. It seems that this grep is
broken, if a test leaves  $exclude undefined, causing it to filter
out all results.

Regards,
Daniel
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