build: check whether hwaccels are enabled in FFmpeg

FFmpeg can be compiled with them disabled, and then it won't provide the
public headers specific to these APIs, causing mpv compilation failure.

(cherry picked from commit baa354c64c)

wscript

@@ -691,7 +691,7 @@ hwaccel_features = [
         'name': '--vaapi-hwaccel',
         'desc': 'libavcodec VAAPI hwaccel',
         'deps': [ 'vaapi' ],
-        'func': check_true,
+        'func': check_statement('libavcodec/vaapi.h', '', use='libav'),
     } , {
         'name': '--vda-hwaccel',
         'desc': 'libavcodec VDA hwaccel',
@@ -710,12 +710,14 @@ hwaccel_features = [
         'name': '--vdpau-hwaccel',
         'desc': 'libavcodec VDPAU hwaccel',
         'deps': [ 'vdpau' ],
-        'func': check_true,
+        'func': check_statement('libavcodec/vdpau.h',
+                                'av_vdpau_alloc_context()',
+                                use='libav'),
     }, {
         'name': '--dxva2-hwaccel',
         'desc': 'libavcodec DXVA2 hwaccel',
         'deps': [ 'gdi' ],
-        'func': check_true,
+        'func': check_statement('libavcodec/dxva2.h', '', use='libav'),
     }
 ]