[PATCH 3/3] autoconf: refine quadrigraph test in _AC_DEFINE_UNQUOTED

It is a very common case that a quadrigraph appears in the argument of
_AC_DEFINE_UNQUOTED, because "#define" is expanded through a quadrigraph.
Therefore, restrict the quadrigraph tests to "$" (for "$(" and "${")
and "(" (for "$(").

At the same time, "#" should not be used inside AC_ECHO because it confuses
m4's comment parsing.  This pre-existing latent bug is caught by test 251:

   AC_DEFINE_UNQUOTED([bar], [[%!_!# X]])

Previously the quadrigraph in "@%:@define bar %!_!# X" made Autoconf fall back
to cat anyway.  Now that Autoconf is not fooled by the quadrigraph, the test
catches that "#" is not special-cased.  Kudos to Eric for coming up with it!

* lib/autoconf/general (_AC_DEFINE_UNQUOTED): Do not blindly
use cat on all quadrigraphs.

Signed-off-by: Paolo Bonzini <[email protected]>
---
 lib/autoconf/general.m4 | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/lib/autoconf/general.m4 b/lib/autoconf/general.m4
index c15fb13..ef6285d 100644
--- a/lib/autoconf/general.m4
+++ b/lib/autoconf/general.m4
@@ -2167,9 +2167,11 @@ m4_define([AC_DEFINE_UNQUOTED], [_AC_DEFINE_Q([_$0], $@)])
 # Append the pre-expanded STRING and a newline to confdefs.h, as if
 # with an unquoted here-doc, but avoiding a fork in the common case of
 # no backslash, no command substitution, no complex variable
-# substitution, and no quadrigraphs.
+# substitution (taking into account quadrigraphs as well).  Also
+# avoid AS_ECHO if "#" is present to avoid confusing m4 with comments,
+# but quadrigraphs are fine in that case.
 m4_define([_AC_DEFINE_UNQUOTED],
-[m4_if(m4_bregexp([$1], [\\\|`\|\$(\|\${\|@]), [-1],
+[m4_if(m4_bregexp([$1], [#\|\\\|`\|\(\$\|@S|@\)\((|{|@{:@\)]), [-1],
        [AS_ECHO(["AS_ESCAPE([$1], [""])"]) >>confdefs.h],
        [cat >>confdefs.h <<_ACEOF
 [$1]
-- 
2.7.4