#4 wxrc should be detected depending on --with-wx... variables

[Tobias Schlemmer]

Original Submission:
wxrc is used to provide prototypes for the wx classes, which are described using xrc. It won't be found if it is not in the search path. As wx-config can be used without being installed, wxrc should be found there, too.

This meens, that configure should find the full absolute path of wxrc according to the given prefix, the given exec-prefix or the given uninstalled build.