Make librfxcodec a subpackage

Since libpainter is going to become a subpackage, we can make librfxcodec
a subpackage as well.

With this change, librfxcodec is configured, built and linked to xrdp
automatically if enabled by "--enable-rfxcodec"

librfxcodec is packaged by "make dist" regardless of "--enable-rfxcodec"

librfxcodec/confgure is run unconditionally, it's needed for "make dist"
to work. But librfxcodec is only compiled if enabled.

Makefile.am

@@ -16,6 +16,12 @@ else
 XRDPVRDIR =
 endif
 
+if XRDP_RFXCODEC
+RFXCODECDIR = librfxcodec
+else
+RFXCODECDIR =
+endif
+
 SUBDIRS = \
   common \
   vnc \
@@ -24,6 +30,7 @@ SUBDIRS = \
   mc \
   $(NEUTRINORDPDIR) \
   libxrdp \
+  $(RFXCODECDIR) \
   xrdp \
   sesman \
   keygen \