decklink: fix flags

-I must be set in CPPFLAGS
Fixes the following message in configure:
checking DeckLinkAPIDispatch.cpp usability... yes
checking DeckLinkAPIDispatch.cpp presence... no

configure.ac

@@ -1760,10 +1760,10 @@ if test "${enable_decklink}" != "no"
 then
   if test "${with_decklink_sdk}" != "no" -a -n "${with_decklink_sdk}"
   then
-    VLC_ADD_CXXFLAGS([decklink decklinkoutput],[-I${with_decklink_sdk}/include])
+    VLC_ADD_CPPFLAGS([decklink decklinkoutput],[-I${with_decklink_sdk}/include])
   fi
   VLC_SAVE_FLAGS
-  CXXFLAGS="${CXXFLAGS} ${CXXFLAGS_decklink}"
+  CPPFLAGS="${CPPFLAGS} ${CPPFLAGS_decklink}"
   AC_LANG_PUSH(C++)
   AC_CHECK_HEADERS(DeckLinkAPIDispatch.cpp, [
       have_decklink=yes

modules/access/Modules.am

@@ -64,7 +64,7 @@ SOURCES_stream_filter_rar = rar/rar.c rar/rar.h rar/stream.c
 
 if HAVE_DECKLINK
 libdecklink_plugin_la_SOURCES = decklink.cpp
-libdecklink_plugin_la_CXXFLAGS = $(AM_CFLAGS) $(CXXFLAGS_decklink)
+libdecklink_plugin_la_CXXFLAGS = $(AM_CFLAGS) $(CPPFLAGS_decklink)
 libdecklink_plugin_la_LIBADD = $(AM_LIBADD) $(LIBS_decklink) -ldl
 libvlc_LTLIBRARIES += libdecklink_plugin.la
 endif

modules/video_output/Modules.am

@@ -12,7 +12,7 @@ SOURCES_android_surface = androidsurface.c
 
 if HAVE_DECKLINK
 libdecklinkoutput_plugin_la_SOURCES = decklink.cpp
-libdecklinkoutput_plugin_la_CXXFLAGS = $(AM_CFLAGS) $(CXXFLAGS_decklinkoutput)
+libdecklinkoutput_plugin_la_CXXFLAGS = $(AM_CFLAGS) $(CPPFLAGS_decklinkoutput)
 libdecklinkoutput_plugin_la_LIBADD = $(AM_LIBADD) $(LIBS_decklink) $(LIBDL)
 libvlc_LTLIBRARIES += libdecklinkoutput_plugin.la
 endif