diff --git a/configure.ac b/configure.ac
index a09d6417..ce7985ff 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1,5 +1,5 @@
 # To compile this into a configure script, you need:
-# * Autoconf 2.50 or newer
+# * Autoconf 2.59c or newer
 # * Gtk (for $prefix/share/aclocal/gtk.m4)
 # * Automake (for aclocal)
 # If you've got them, running "autoreconf" should work.
@@ -17,6 +17,8 @@ AC_PROG_RANLIB
 ifdef([AM_PROG_AR],[AM_PROG_AR])
 AM_PROG_CC_C_O
 
+AC_PROG_CC_C99
+
 # Mild abuse of the '--enable' option format to allow manual
 # specification of setuid or setgid setup in pterm.
 setidtype=none
diff --git a/mkfiles.pl b/mkfiles.pl
index de68fd0e..9427db65 100755
--- a/mkfiles.pl
+++ b/mkfiles.pl
@@ -627,8 +627,8 @@ if (defined $makefiles{'cygwin'}) {
     "# You may also need to tell windres where to find include files:\n".
     "# RCINC = --include-dir c:\\cygwin\\include\\\n".
     "\n".
-    &splitline("CFLAGS = -Wall -O2 -D_WINDOWS -DDEBUG -DWIN32S_COMPAT".
-      " -D_NO_OLDNAMES " .
+    &splitline("CFLAGS = -Wall -O2 -std=gnu99 -D_WINDOWS -DDEBUG".
+      " -DWIN32S_COMPAT -D_NO_OLDNAMES " .
 	       (join " ", map {"-I$dirpfx$_"} @srcdirs)) .
 	       "\n".
     "LDFLAGS = -s\n".
@@ -1420,7 +1420,7 @@ if (defined $makefiles{'gtk'}) {
     "\n".
     "unexport CFLAGS # work around a weird issue with krb5-config\n".
     "\n".
-    &splitline("CFLAGS = -O2 -Wall -Werror -g " .
+    &splitline("CFLAGS = -O2 -Wall -Werror -std=gnu99 -g " .
 	       (join " ", map {"-I$dirpfx$_"} @srcdirs) .
 	       " \$(shell \$(GTK_CONFIG) --cflags)").
 		 " -D _FILE_OFFSET_BITS=64\n".
@@ -1501,7 +1501,7 @@ if (defined $makefiles{'unix'}) {
     "\n".
     "unexport CFLAGS # work around a weird issue with krb5-config\n".
     "\n".
-    &splitline("CFLAGS = -O2 -Wall -Werror -g " .
+    &splitline("CFLAGS = -O2 -Wall -Werror -std=gnu99 -g " .
 	       (join " ", map {"-I$dirpfx$_"} @srcdirs)).
 		 " -D _FILE_OFFSET_BITS=64\n".
     "ULDFLAGS = \$(LDFLAGS)\n".
@@ -1743,7 +1743,7 @@ if (defined $makefiles{'osx'}) {
     print
     "CC = \$(TOOLPATH)gcc\n".
     "\n".
-    &splitline("CFLAGS = -O2 -Wall -Werror -g " .
+    &splitline("CFLAGS = -O2 -Wall -Werror -std=gnu99 -g " .
 	       (join " ", map {"-I$dirpfx$_"} @srcdirs))."\n".
     "MLDFLAGS = -framework Cocoa\n".
     "ULDFLAGS =\n".