decklink: don't force link with pthread

Only the Linux and macOS targets probably require it. It should not be the case
on Windows. BlackMagic probably doesn't support other platforms than these 3.

For Linux we already build with -pthread. For macOS this is not even needed.

modules/access/Makefile.am

@@ -132,7 +132,7 @@ if HAVE_WIN32
 libdecklink_plugin_la_LIBADD += $(LIBCOM)
 endif
 if HAVE_LINUX
-libdecklink_plugin_la_LIBADD += $(LIBDL) -lpthread
+libdecklink_plugin_la_LIBADD += $(LIBDL)
 endif
 if HAVE_DECKLINK
 access_LTLIBRARIES += libdecklink_plugin.la

modules/stream_out/Makefile.am

@@ -59,7 +59,7 @@ if HAVE_WIN32
 libstream_out_sdi_plugin_la_LIBADD += $(LIBCOM)
 endif
 if HAVE_LINUX
-libstream_out_sdi_plugin_la_LIBADD += $(LIBDL) -lpthread
+libstream_out_sdi_plugin_la_LIBADD += $(LIBDL)
 endif
 libstream_out_sdi_plugin_la_SOURCES = stream_out/sdi/sdiout.cpp \
         stream_out/sdi/sdiout.hpp \

modules/video_output/Makefile.am

@@ -27,7 +27,7 @@ if HAVE_WIN32
 libdecklinkoutput_plugin_la_LIBADD += $(LIBCOM)
 endif
 if HAVE_LINUX
-libdecklinkoutput_plugin_la_LIBADD += $(LIBDL) -lpthread
+libdecklinkoutput_plugin_la_LIBADD += $(LIBDL)
 endif
 vout_LTLIBRARIES += libdecklinkoutput_plugin.la
 endif