Return type of functions must be explicit, C99
https://bugs.gentoo.org/886461
--- a/src/vf2c/main.c
+++ b/src/vf2c/main.c
@@ -110,9 +110,9 @@
 #endif
 
 #ifdef KR_headers
-main(argc, argv) int argc; char **argv;
+int main(argc, argv) int argc; char **argv;
 #else
-main(int argc, char **argv)
+int main(int argc, char **argv)
 #endif
 {
 xargc = argc;