configure: Add fake prototypes for C99 compatibility

The xxmc-related configure probes assume that the compiler
supports implicit function declarations because it tries to
call the functions without including the appropriate headers,
for link testing.

As the headers are not determined yet at this point, use
a fake prototype (the same that autoconf uses) to avoid
the implicit function declarations.

This avoids altering the outcome of these checks with future
compilers which do not support implicit function declarations.

Submitted upstream:

  <https://sourceforge.net/p/xine/xine-lib-1.2/merge-requests/2/>

diff --git a/configure b/configure
index a4009e857777b5cf..3a83b40efda8fd5d 100755
--- a/configure
+++ b/configure
@@ -28563,7 +28563,7 @@ $as_echo "" >&6; }
             LIBS="$XXMC_LIBS $X_LIBS $XV_LIBS $LIBS"
             cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
-
+char XvMCPutSlice(void);
 int
 main ()
 {
@@ -28578,7 +28578,7 @@ else
   LIBS="$XXMC_LIBS -lXvMC $X_LIBS $XV_LIBS $LIBS $DYNAMIC_LD_LIBS"
                             cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
-
+char XvMCPutSlice(void);
 int
 main ()
 {
@@ -28616,7 +28616,7 @@ done
             else
                 cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
-
+char XvMCCreateContext(void);
 int
 main ()
 {
@@ -28631,7 +28631,7 @@ else
   LIBS="$XXMC_LIBS -lXvMC $X_LIBS $XV_LIBS $LIBS"
                                 cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
-
+char XvMCCreateContext(void);
 int
 main ()
 {
@@ -28675,7 +28675,7 @@ $as_echo "" >&6; }
             LIBS="$XVMC_LIBS $X_LIBS $XV_LIBS $LIBS"
             cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
-
+char XvMCCreateContext(void);
 int
 main ()
 {
@@ -28690,7 +28690,7 @@ else
   LIBS="$XVMC_LIBS -lXvMC $X_LIBS $XV_LIBS $LIBS $DYNAMIC_LD_LIBS"
                             cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
-
+char XvMCCreateContext(void);
 int
 main ()
 {
diff --git a/m4/video_out.m4 b/m4/video_out.m4
index 150b477697297c03..8aa1f4a3b9267ff9 100644
--- a/m4/video_out.m4
+++ b/m4/video_out.m4
@@ -496,9 +496,9 @@ AC_DEFUN([XINE_VIDEO_OUT_PLUGINS], [
             AC_MSG_CHECKING([whether to enable the xxmc plugin with VLD extensions])
             AC_MSG_RESULT([])
             LIBS="$XXMC_LIBS $X_LIBS $XV_LIBS $LIBS"
-            AC_LINK_IFELSE([AC_LANG_PROGRAM([[]], [[XvMCPutSlice()]])], [have_xxmc=yes],
+            AC_LINK_IFELSE([AC_LANG_PROGRAM([[char XvMCPutSlice(void);]], [[XvMCPutSlice()]])], [have_xxmc=yes],
                            [LIBS="$XXMC_LIBS -lXvMC $X_LIBS $XV_LIBS $LIBS $DYNAMIC_LD_LIBS"
-                            AC_LINK_IFELSE([AC_LANG_PROGRAM([[]], [[XvMCPutSlice()]])],
+                            AC_LINK_IFELSE([AC_LANG_PROGRAM([[char XvMCPutSlice(void);]], [[XvMCPutSlice()]])],
                                            [have_xxmc=yes XXMC_LIBS="$XXMC_LIBS -lXvMC"])])
             if test x"$have_xxmc" = x"yes"; then
                 AC_CHECK_HEADERS([X11/extensions/vldXvMC.h],
@@ -506,9 +506,9 @@ AC_DEFUN([XINE_VIDEO_OUT_PLUGINS], [
                                   AC_DEFINE([HAVE_VLDXVMC], 1, [Define if you have vldXvMC.h])],
                                   [have_vldexts=no])
             else
-                AC_LINK_IFELSE([AC_LANG_PROGRAM([[]], [[XvMCCreateContext()]])], [have_xxmc=yes],
+                AC_LINK_IFELSE([AC_LANG_PROGRAM([[char XvMCCreateContext(void);]], [[XvMCCreateContext()]])], [have_xxmc=yes],
                                [LIBS="$XXMC_LIBS -lXvMC $X_LIBS $XV_LIBS $LIBS"
-                                AC_LINK_IFELSE([AC_LANG_PROGRAM([[]], [[XvMCCreateContext()]])],
+                                AC_LINK_IFELSE([AC_LANG_PROGRAM([[char XvMCCreateContext(void);]], [[XvMCCreateContext()]])],
                                                [have_xxmc=yes XXMC_LIBS="$XXMC_LIBS -lXvMC"])])
             fi
             if test x"$have_xxmc" = x"yes"; then
@@ -521,9 +521,9 @@ AC_DEFUN([XINE_VIDEO_OUT_PLUGINS], [
             AC_MSG_CHECKING([whether to enable the xvmc plugin])
             AC_MSG_RESULT([])
             LIBS="$XVMC_LIBS $X_LIBS $XV_LIBS $LIBS"
-            AC_LINK_IFELSE([AC_LANG_PROGRAM([[]], [[XvMCCreateContext()]])], [have_xvmc=yes],
+            AC_LINK_IFELSE([AC_LANG_PROGRAM([[char XvMCCreateContext(void);]], [[XvMCCreateContext()]])], [have_xvmc=yes],
                            [LIBS="$XVMC_LIBS -lXvMC $X_LIBS $XV_LIBS $LIBS $DYNAMIC_LD_LIBS"
-                            AC_LINK_IFELSE([AC_LANG_PROGRAM([[]], [[XvMCCreateContext()]])],
+                            AC_LINK_IFELSE([AC_LANG_PROGRAM([[char XvMCCreateContext(void);]], [[XvMCCreateContext()]])],
                                            [have_xvmc=yes XVMC_LIBS="$XVMC_LIBS -lXvMC"])])
             if test x"$have_xvmc" = x"yes"; then
                 AC_CHECK_HEADERS([X11/extensions/XvMC.h], [], [have_xvmc=no])
