Include <stdlib.h> for the exit function.  This avoids a failing
GNUPG_CHECK_IPC check in case the compiler no longer supports implicit
function declarations.

Explicitly declare the return type of main as int.  This too avoids
failures with future compilers.

Submitted upstream:

  <https://lists.gnupg.org/pipermail/gnupg-devel/2023-January/035264.html>

diff --git a/acinclude.m4 b/acinclude.m4
index 44dfd85..ed4992e 100644
--- a/acinclude.m4
+++ b/acinclude.m4
@@ -314,6 +314,7 @@ define(GNUPG_CHECK_IPC,
              #include <sys/types.h>
              #include <sys/ipc.h>
              #include <sys/shm.h>
+             #include <stdlib.h>
              int main()
              {
                int id;
diff --git a/configure.ac b/configure.ac
index 111271e..5aa4203 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1271,7 +1271,7 @@ if test "$use_regex" = yes ; then
        AC_TRY_RUN([
 #include <unistd.h>
 #include <regex.h>
-main() { regex_t blah ; regmatch_t p; p.rm_eo = p.rm_eo; return regcomp(&blah, "foo.*bar", REG_NOSUB) || regexec (&blah, "foobar", 0, NULL, 0); }],
+int main(void) { regex_t blah ; regmatch_t p; p.rm_eo = p.rm_eo; return regcomp(&blah, "foo.*bar", REG_NOSUB) || regexec (&blah, "foobar", 0, NULL, 0); }],
        gnupg_cv_regex_broken=no, gnupg_cv_regex_broken=yes, gnupg_cv_regex_broken=yes))
 
      if test $gnupg_cv_regex_broken = yes ; then