Add a variable for wx-config command.

We know that BUILD is "debug" or "release" (checked at the top of the
Makefile).

Preparation for using the system wx-config if WxWidgets has not been built.
Only call wx-config for Mana Shell executable and objects in Makefile.

Does not really matter now since ManaShell is the only application in the
Makefile.
Use system wxWidgets on Linux if custom build is not available.

Not particularly fond of the naming of "WX_CONFIG_CUSTOM" but have no ohter
ideas. Thought about appending the name of the variable that points to the
custom build directory but that does not really work now when it is
"WXWIDGETS". Changing the name of this variable would disrupt build
environments people have already set up so leave it as is for now.
Modify Linux build instructions.

To mention that system wxWidgets is used if wx-config for custom build
cannot be found.