configure: remove use of RETSIGTYPE

This was previously defined by the obsolete AC_TYPE_SIGNAL macro,
which was removed in 2682e5f5. The deprecation text says

> Your code may safely assume C89 semantics that RETSIGTYPE is void.

So, remove it and just use void instead.

Closes #6861

CMake/CurlTests.c

@@ -5,7 +5,7 @@
  *                            | (__| |_| |  _ <| |___
  *                             \___|\___/|_| \_\_____|
  *
- * Copyright (C) 1998 - 2020, Daniel Stenberg, <daniel@haxx.se>, et al.
+ * Copyright (C) 1998 - 2021, Daniel Stenberg, <daniel@haxx.se>, et al.
  *
  * This software is licensed as described in the file COPYING, which
  * you should have received as part of this distribution. The terms
@@ -214,24 +214,6 @@ if (sizeof (bool *) )
 #include <float.h>
 int main() { return 0; }
 #endif
-#ifdef RETSIGTYPE_TEST
-#include <sys/types.h>
-#include <signal.h>
-#ifdef signal
-# undef signal
-#endif
-#ifdef __cplusplus
-extern "C" void (*signal (int, void (*)(int)))(int);
-#else
-void (*signal ()) ();
-#endif
-
-int
-main ()
-{
-  return 0;
-}
-#endif
 #ifdef HAVE_INET_NTOA_R_DECL
 #include <arpa/inet.h>
 

CMake/Platforms/WindowsCache.cmake

@@ -132,7 +132,6 @@ if(NOT UNIX)
       set(HAVE_GETADDRINFO 0)
     endif()
     set(STDC_HEADERS 1)
-    set(RETSIGTYPE_TEST 1)
 
     set(HAVE_SIGACTION 0)
     set(HAVE_MACRO_SIGSETJMP 0)