Add strict-prototypes warning if supported.

Ensure that all functions are correctly declared for c.
There was one function in libsrtp that was not declared correctly
and now it can be caught.

This flags needed a custom program to check in configure, the built
in function was invalid when using this flag.
diff --git a/configure b/configure
index 4a08225..776159d 100755
--- a/configure
+++ b/configure
@@ -4812,8 +4812,8 @@
 WERROR=""
 for w in -Werror -errwarn; do
   if test "x$WERROR" = "x"; then
-    { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether the compiler supports $w" >&5
-$as_echo_n "checking whether the compiler supports $w... " >&6; }
+    { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether ${CC-c} accepts $w" >&5
+$as_echo_n "checking whether ${CC-c} accepts $w... " >&6; }
     save_cflags="$CFLAGS"
     if test "x$CFLAGS" = "x"; then :
   CFLAGS="$w"
@@ -4822,14 +4822,7 @@
 fi
     cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
-
-int
-main ()
-{
-
-  ;
-  return 0;
-}
+int main(void) { return 0; }
 _ACEOF
 if ac_fn_c_try_compile "$LINENO"; then :
   WERROR="$w"
@@ -4844,8 +4837,8 @@
   fi
 done
 
-{ $as_echo "$as_me:${as_lineno-$LINENO}: checking whether the compiler supports -fPIC" >&5
-$as_echo_n "checking whether the compiler supports -fPIC... " >&6; }
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking whether ${CC-c} accepts -fPIC" >&5
+$as_echo_n "checking whether ${CC-c} accepts -fPIC... " >&6; }
 save_cflags="$CFLAGS"
 if test "x$CFLAGS" = "x"; then :
   CFLAGS="-fPIC"
@@ -4854,14 +4847,7 @@
 fi
 cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
-
-int
-main ()
-{
-
-  ;
-  return 0;
-}
+int main(void) { return 0; }
 _ACEOF
 if ac_fn_c_try_compile "$LINENO"; then :
   if test "x$supported_cflags" = "x"; then :
@@ -4884,9 +4870,9 @@
 rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
 
 if test "$EMPTY_CFLAGS" = "yes"; then
-  for f in -Wall -pedantic; do
-    { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether the compiler supports $f" >&5
-$as_echo_n "checking whether the compiler supports $f... " >&6; }
+  for f in -Wall -pedantic -Wstrict-prototypes; do
+    { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether ${CC-c} accepts $f" >&5
+$as_echo_n "checking whether ${CC-c} accepts $f... " >&6; }
     save_cflags="$CFLAGS"
     if test "x$CFLAGS" = "x"; then :
   CFLAGS="$f"
@@ -4895,14 +4881,7 @@
 fi
     cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
-
-int
-main ()
-{
-
-  ;
-  return 0;
-}
+int main(void) { return 0; }
 _ACEOF
 if ac_fn_c_try_compile "$LINENO"; then :
   if test "x$supported_cflags" = "x"; then :
@@ -4923,8 +4902,8 @@
   OOPT=""
   for f in -O4 -O3; do
     if test "x$OOPT" = "x"; then
-      { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether the compiler supports $f" >&5
-$as_echo_n "checking whether the compiler supports $f... " >&6; }
+      { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether ${CC-c} accepts $f" >&5
+$as_echo_n "checking whether ${CC-c} accepts $f... " >&6; }
       save_cflags="$CFLAGS"
       if test "x$CFLAGS" = "x"; then :
   CFLAGS="$f"
@@ -4933,14 +4912,7 @@
 fi
       cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
-
-int
-main ()
-{
-
-  ;
-  return 0;
-}
+int main(void) { return 0; }
 _ACEOF
 if ac_fn_c_try_compile "$LINENO"; then :
   if test "x$supported_cflags" = "x"; then :
@@ -4961,8 +4933,8 @@
   done
 
   for f in -fexpensive-optimizations -funroll-loops; do
-    { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether the compiler supports $f" >&5
-$as_echo_n "checking whether the compiler supports $f... " >&6; }
+    { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether ${CC-c} accepts $f" >&5
+$as_echo_n "checking whether ${CC-c} accepts $f... " >&6; }
     save_cflags="$CFLAGS"
     if test "x$CFLAGS" = "x"; then :
   CFLAGS="$f"
@@ -4971,14 +4943,7 @@
 fi
     cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
-
-int
-main ()
-{
-
-  ;
-  return 0;
-}
+int main(void) { return 0; }
 _ACEOF
 if ac_fn_c_try_compile "$LINENO"; then :
   if test "x$supported_cflags" = "x"; then :
@@ -4998,8 +4963,8 @@
 fi
 
 for f in -Wno-language-extension-token; do
-  { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether the compiler supports $f" >&5
-$as_echo_n "checking whether the compiler supports $f... " >&6; }
+  { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether ${CC-c} accepts $f" >&5
+$as_echo_n "checking whether ${CC-c} accepts $f... " >&6; }
   save_cflags="$CFLAGS"
   testf=$(echo "$f" | $SED 's|-Wno-\(.*\)|-W\1|g')
   if test "x$CFLAGS" = "x"; then :
@@ -5009,14 +4974,7 @@
 fi
   cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
-
-int
-main ()
-{
-
-  ;
-  return 0;
-}
+int main(void) { return 0; }
 _ACEOF
 if ac_fn_c_try_compile "$LINENO"; then :
   if test "x$supported_cflags" = "x"; then :
@@ -5536,24 +5494,23 @@
 /* end confdefs.h.  */
 
 #include <winsock2.h>
-
-int
-main ()
+int main(void)
 {
-
-socket(0, 0, 0);
-
-  ;
-  return 0;
+    int fd = socket(0, 0, 0);
+    if (fd < 0)
+      return -1;
+    else
+      return 0;
 }
+
 _ACEOF
 if ac_fn_c_try_link "$LINENO"; then :
   ac_cv_func_socket=yes
-    { $as_echo "$as_me:${as_lineno-$LINENO}: result: yes" >&5
+     { $as_echo "$as_me:${as_lineno-$LINENO}: result: yes" >&5
 $as_echo "yes" >&6; }
 else
   LIBS="$SAVELIBS"
-    { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+     { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
 $as_echo "no" >&6; }
 fi
 rm -f core conftest.err conftest.$ac_objext \