Guard against redef of asciidoctor in common b2.

doc/asciidoctor.jam

@@ -9,6 +9,8 @@ import scanner ;
 import generators ;
 import boostbook ;
 
+if ! [ type.registered ASCIIDOC ]
+{
     # File type
     
     type.register ASCIIDOC : asciidoc adoc ;
@@ -25,6 +27,7 @@ class asciidoc-scanner : common-scanner
     
     scanner.register asciidoc-scanner : include ;
     type.set-scanner ASCIIDOC : asciidoc-scanner ;
+}
 
 # Define generators