build: change vf_dlopen test

Didn't work on Windows. Apparently, WIN32 is not set in the Makefile.

Makefile

@@ -116,10 +116,7 @@ ifeq ($(HAVE_AVUTIL_REFCOUNTING),no)
     SOURCES-yes                 += video/decode/lavc_dr1.c
 endif
 
-SOURCES-$(DL)                   += video/filter/vf_dlopen.c
-ifeq ($(DL),no)
-    SOURCES-$(WIN32)            += video/filter/vf_dlopen.c
-endif
+SOURCES-$(DLOPEN)               += video/filter/vf_dlopen.c
 
 SOURCES = talloc.c \
           audio/audio.c \

configure

@@ -2842,6 +2842,18 @@ fi
 echores "$_encoding"
 
 
+# needs dlopen on unix, uses winapi on windows
+_dlopen="$_dl"
+if win32 ; then
+  _dlopen=yes
+fi
+
+if test "$_dlopen" = yes ; then
+  def_dlopen='#define CONFIG_DLOPEN 1'
+else
+  def_dlopen='#undef CONFIG_DLOPEN'
+fi
+
 #############################################################################
 
 echocheck "compiler support for noexecstack"
@@ -2966,6 +2978,7 @@ COREAUDIO = $_coreaudio
 COREVIDEO = $_corevideo
 DIRECT3D = $_direct3d
 DL = $_dl
+DLOPEN = $_dlopen
 SDL = $_sdl
 SDL2 = $_sdl2
 DSOUND = $_dsound
@@ -3126,6 +3139,7 @@ $def_libavfilter
 $def_vf_lavfi
 $def_af_lavfi
 
+$def_dlopen
 
 /* Audio output drivers */
 $def_alsa

video/filter/vf.c

@@ -109,7 +109,7 @@ static const vf_info_t *const filter_list[] = {
     &vf_info_sub,
     &vf_info_yadif,
     &vf_info_stereo3d,
-#if defined(HAVE_LIBDL) || defined(_WIN32)
+#ifdef CONFIG_DLOPEN
     &vf_info_dlopen,
 #endif
     NULL