[PATCH][REVIEW] gbuild conversion: idlc module

[David Ostrovsky]

Hi,

this is gbuild conversion for idlc module.

We don't have lexer function for gbuild, so I converted dmake stuff 
directly in CustomTarget_scanner.mk.
@Matus, thank you for your help on IRC with that.
If we have more lexer stuff it would be nice to see a convenience 
function gb_Executable_add_lexers (a lá gb_Executable_add_grammars)
though.

David