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.

https://bugs.gentoo.org/651522
Remove the insane check for isinf(), since every sane compiler nowadays supports it.

--- a/configure.in
+++ b/configure.in
@@ -183,7 +183,7 @@
                   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}"
@@ -378,28 +378,9 @@
 
 dnl Platform-specific fixes.
 dnl 
-
-dnl Sun Forte C provides isinf() in the unbundled -lsunmath. Solaris
-dnl declares finite() in ieeefp.h. It's preferrable not to link 
-dnl to -lsunmath as it's not present on all machines.
-have_isinf=no
-AC_CHECK_HEADERS(math.h,
-        AC_CHECK_LIB(m,isinf,
-                have_isinf=yes))
-
-if test "$have_isinf" = "no"; then
-    AC_CHECK_HEADER(ieeefp.h,
-             [AC_CHECK_FUNC(finite, 
-	               AC_DEFINE(HAVE_IEEEFP_H,1,
-		                 [Define if finite() is defined in ieeefp.h]),
-                       AC_MSG_ERROR([Can't find a definition of neither 
-                                     finite nor isinf]))],
-             [AC_MSG_ERROR(Can't find neither a definition of
-	                   isinf nor ieeefp.h)])
-else
-    AC_DEFINE([HAVE_ISINF], 1, 
-              [Define if the isinf() function is available])
-fi
+dnl every modern compiler supports C99's isinf()
+AC_DEFINE([HAVE_ISINF], 1, 
+          [Define if the isinf() function is available])
 
 dnl Alpha needs -mieee or we get a segfault
 dnl This shouldn't be needed but is here until a real solution is found