https://bugs.gentoo.org/870679
https://bugs.gentoo.org/871279

Generated file and configure.in is written for autoconf 2.13.
--- a/configure
+++ b/configure
@@ -752,5 +752,5 @@
 #include "confdefs.h"
 
-main(){return(0);}
+int main(void){return(0);}
 EOF
 if { (eval echo configure:757: \"$ac_link\") 1>&5; (eval $ac_link) 2>&5; } && test -s conftest${ac_exeext}; then
@@ -1356,4 +1356,5 @@
 #include "confdefs.h"
 
+void ${x_direct_test_function}(void);
 int main() {
 ${x_direct_test_function}()
@@ -2440,4 +2441,5 @@
 #include "confdefs.h"
 
+void XpmReadFileToPixmap(void);
 int main() {
 XpmReadFileToPixmap()