Document RDP compositor options

Help was missing for the RDP compositor, this patch gives details
about available options.
diff --git a/configure.ac b/configure.ac
index 0864d10..1f5014a 100644
--- a/configure.ac
+++ b/configure.ac
@@ -197,6 +197,7 @@
 AM_CONDITIONAL([ENABLE_RDP_COMPOSITOR],
                [test x$enable_rdp_compositor = xyes])
 if test x$enable_rdp_compositor = xyes; then
+  AC_DEFINE([BUILD_RDP_COMPOSITOR], [1], [Build the RDP compositor])
   PKG_CHECK_MODULES(RDP_COMPOSITOR, [freerdp >= 1.1.0])
 fi