gcc: Add note about the dependency on Bison.

* gcc-plugin/README: Add.

gcc-plugin/README

@@ -8,3 +8,5 @@ tasks.
 Plug-ins are supported starting from GCC 4.5.
 
 To run the test suite, GNU Guile 1.8.x or 2.0.x is needed.
+
+When building from SVN, GNU Bison 2.5+ is required.