Enable more single-thread optimizations in gnulib code

On many systems (esp. BSD ones), building a recent m4 snapshot produces these
warnings:

--------------------------------------------------------------------------------
  CC       regex.o
In file included from ../../lib/regex_internal.h:57:0,
                 from ../../lib/regex.c:70:
../../lib/regcomp.c: In function 'rpl_regfree':
../../lib/glthread/lock.h:640:38: warning: statement with no effect [-Wunused-value]
 # define glthread_lock_destroy(NAME) 0
                                      ^
../../lib/regex_internal.h:60:26: note: in expansion of macro 'glthread_lock_destroy'
 # define lock_fini(lock) glthread_lock_destroy (&(lock))
                          ^
../../lib/regcomp.c:638:7: note: in expansion of macro 'lock_fini'
       lock_fini (dfa->lock);
       ^
../../lib/regcomp.c: In function 're_compile_internal':
../../lib/glthread/lock.h:640:38: warning: statement with no effect [-Wunused-value]
 # define glthread_lock_destroy(NAME) 0
                                      ^
../../lib/regex_internal.h:60:26: note: in expansion of macro 'glthread_lock_destroy'
 # define lock_fini(lock) glthread_lock_destroy (&(lock))
                          ^
../../lib/regcomp.c:781:7: note: in expansion of macro 'lock_fini'
       lock_fini (dfa->lock);
       ^
../../lib/glthread/lock.h:640:38: warning: statement with no effect [-Wunused-value]
 # define glthread_lock_destroy(NAME) 0
                                      ^
../../lib/regex_internal.h:60:26: note: in expansion of macro 'glthread_lock_destroy'
 # define lock_fini(lock) glthread_lock_destroy (&(lock))
                          ^
../../lib/regcomp.c:814:7: note: in expansion of macro 'lock_fini'
       lock_fini (dfa->lock);
       ^
../../lib/regexec.c: In function 'rpl_regexec':
../../lib/glthread/lock.h:638:35: warning: statement with no effect [-Wunused-value]
 # define glthread_lock_lock(NAME) 0
                                   ^
../../lib/regex_internal.h:61:26: note: in expansion of macro 'glthread_lock_lock'
 # define lock_lock(lock) glthread_lock_lock (&(lock))
                          ^
../../lib/regexec.c:214:3: note: in expansion of macro 'lock_lock'
   lock_lock (dfa->lock);
   ^
../../lib/glthread/lock.h:639:37: warning: statement with no effect [-Wunused-value]
 # define glthread_lock_unlock(NAME) 0
                                     ^
../../lib/regex_internal.h:62:28: note: in expansion of macro 'glthread_lock_unlock'
 # define lock_unlock(lock) glthread_lock_unlock (&(lock))
                            ^
../../lib/regexec.c:221:3: note: in expansion of macro 'lock_unlock'
   lock_unlock (dfa->lock);
   ^
In file included from ../../lib/regex.c:74:0:
../../lib/regexec.c:198:13: warning: unused variable 'dfa' [-Wunused-variable]
   re_dfa_t *dfa = preg->buffer;
             ^
In file included from ../../lib/regex_internal.h:57:0,
                 from ../../lib/regex.c:70:
../../lib/regexec.c: In function 're_search_stub':
../../lib/glthread/lock.h:638:35: warning: statement with no effect [-Wunused-value]
 # define glthread_lock_lock(NAME) 0
                                   ^
../../lib/regex_internal.h:61:26: note: in expansion of macro 'glthread_lock_lock'
 # define lock_lock(lock) glthread_lock_lock (&(lock))
                          ^
../../lib/regexec.c:391:3: note: in expansion of macro 'lock_lock'
   lock_lock (dfa->lock);
   ^
../../lib/glthread/lock.h:639:37: warning: statement with no effect [-Wunused-value]
 # define glthread_lock_unlock(NAME) 0
                                     ^
../../lib/regex_internal.h:62:28: note: in expansion of macro 'glthread_lock_unlock'
 # define lock_unlock(lock) glthread_lock_unlock (&(lock))
                            ^
../../lib/regexec.c:455:3: note: in expansion of macro 'lock_unlock'
   lock_unlock (dfa->lock);
   ^
In file included from ../../lib/regex.c:74:0:
../../lib/regexec.c:378:13: warning: unused variable 'dfa' [-Wunused-variable]
   re_dfa_t *dfa = bufp->buffer;
             ^
--------------------------------------------------------------------------------

These warnings are indicating that the regex module is being built for multi-
threading, while at the same time the Gnulib 'lock' module is being optimized
for single-threading (due to the gl_DISABLE_THREADS invocation in configure.ac).

According to the Gnulib documentation section "Optimizations of multithreaded
code" several more optimizations can be enabled. This patch
  - enables these single-threading optimizations,
  - by doing so, gets rid of the warnings in regex.c,
  - causes no test failures.

OK to push?

From 611fa6cd92671318b22e70f931db695a58ecdaf3 Mon Sep 17 00:00:00 2001
From: Bruno Haible <[email protected]>
Date: Thu, 13 May 2021 10:49:13 +0200
Subject: [PATCH] Enable more single-thread optimizations in gnulib code.

* configure.ac (GNULIB_REGEX_SINGLE_THREAD, GNULIB_MBRTOWC_SINGLE_THREAD,
GNULIB_WCHAR_SINGLE_LOCALE): Define as C macros.
---
 configure.ac | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/configure.ac b/configure.ac
index 04a7d4b..517666f 100644
--- a/configure.ac
+++ b/configure.ac
@@ -42,6 +42,9 @@ gl_ASSERT_NO_GNULIB_POSIXCHECK
 
 # M4 is single-threaded; so we can optimize gnulib code by using this:
 gl_DISABLE_THREADS
+AC_DEFINE([GNULIB_REGEX_SINGLE_THREAD], [1], [Define to optimize regex.])
+AC_DEFINE([GNULIB_MBRTOWC_SINGLE_THREAD], [1], [Define to optimize mbrtowc.])
+AC_DEFINE([GNULIB_WCHAR_SINGLE_LOCALE], [1], [Define to optimize mbrtowc.])
 
 AC_ARG_ENABLE([gcc-warnings],
   [AS_HELP_STRING([--enable-gcc-warnings],
-- 
2.7.4