diff --git a/configure b/configure
index 431f938c..0ea723c6 100755
--- a/configure
+++ b/configure
@@ -4844,7 +4844,7 @@ if test $? -eq 1; then echo "the user \"$RUNNING_USER\" does not exists, please
 
 echo; echo
 
-CFLAGS="$static -O2 -Wall -Wextra -Wuninitialized -Wno-format-truncation -g"
+CFLAGS="$static -std=c99 -O2 -Wall -Wextra -Wuninitialized -Wno-format-truncation -g"
 LIBS="$antispam_libs $sunos_libs "
 OBJS="dirs.o base64.o misc.o counters.o cfg.o sig.o decoder.o hash.o parser.o parser_utils.o rules.o smtp.o session.o bdat.o message.o attachment.o digest.o store.o archive.o tai.o import.o import_maildir.o import_mailbox.o import_pop3.o import_imap.o imap.o pop3.o extract.o mydomains.o $objs"
 
diff --git a/configure.in b/configure.in
index 0bdf46d2..077a2845 100644
--- a/configure.in
+++ b/configure.in
@@ -530,7 +530,7 @@ if test $? -eq 1; then echo "the user \"$RUNNING_USER\" does not exists, please
 
 echo; echo
 
-CFLAGS="$static -O2 -Wall -Wextra -Wuninitialized -Wno-format-truncation -g"
+CFLAGS="$static -std=c99 -O2 -Wall -Wextra -Wuninitialized -Wno-format-truncation -g"
 LIBS="$antispam_libs $sunos_libs "
 OBJS="dirs.o base64.o misc.o counters.o cfg.o sig.o decoder.o hash.o parser.o parser_utils.o rules.o smtp.o session.o bdat.o message.o attachment.o digest.o store.o archive.o tai.o import.o import_maildir.o import_mailbox.o import_pop3.o import_imap.o imap.o pop3.o extract.o mydomains.o $objs"