https://bugs.gentoo.org/874729
https://src.fedoraproject.org/rpms/dia/blob/0a14169fc36b959598074065678e0126830317f8/f/dia-configure-c99.patch

C99 removes support for implicit ints, so declare the return type
of main explicitly.  Future compilers are likely to require explicit
type declarations.

Upstream has switched to Meson, and it does not seem to use this
particular check anymore.

--- a/configure.in
+++ b/configure.in
@@ -197,7 +197,7 @@ if test "$png_ok" = yes; then
                   png_structp pp;
 		  png_infop info; 
 		  png_colorp cmap; 
-		  main() { void*foo = png_create_read_struct; }],
+		  int main(void) { void*foo = png_create_read_struct; }],
                  png_ok=yes,
                  png_ok=no)
   LDFLAGS="${old_LDFLAGS}"