Avoid implicit ints for improved C99 compatibility.

Submitted upstream: <https://bz.apache.org/bugzilla/show_bug.cgi?id=66396>

diff --git a/build/apr_common.m4 b/build/apr_common.m4
index 6b5c0f033b9e2aca..26607b8479d9be78 100644
--- a/build/apr_common.m4
+++ b/build/apr_common.m4
@@ -473,7 +473,7 @@ $1
 #else
 #define binmode
 #endif
-main()
+int main(void)
 {
   FILE *f=fopen("conftestval", "w" binmode);
   if (!f) exit(1);
@@ -531,7 +531,7 @@ AC_TRY_RUN([
 #include <errno.h>
 #include <string.h>
 #include <stdio.h>
-main()
+int main(void)
 {
   char buf[1024];
   if (strerror_r(ERANGE, buf, sizeof buf) < 1) {
diff --git a/build/dbm.m4 b/build/dbm.m4
index 57bd131fc3b74099..e8c51b5dc2c92b68 100644
--- a/build/dbm.m4
+++ b/build/dbm.m4
@@ -235,7 +235,7 @@ AC_DEFUN([APU_TRY_BERKELEY_DB],
 #include <stdlib.h>
 #include <stdio.h>
 #include <$apu_try_berkeley_db_header>
-main ()
+int main (void)
 {
   int major, minor, patch;