Due to recent changes, setting xxx_compiler in configure() had no effect.
This commit fixes the problem and changes the examples to show setting
the xxx_compiler variables at the top level of the wscript, so that
the default compilers are correctly reported in the help output.
* Added ldc2 as a separate tool. It can be detected/used with --check-d-compiler in the compiler_d tool now.
* Added code to make sure that the compilers we find actually are the right ones (heuristics).
* Correct some old comments.