Remove the user setting of the USE_LOCALBASE_FOR_X11 definition.

Remove the user setting of the USE_LOCALBASE_FOR_X11 definition.
(Users can still set it if they want, but it won't make much
difference).

In bsd.pkg.mk, check for the presence of the xpkgwedge package to
determine whether to use LOCALBASE for X11 programs.  Set
_USE_LOCALBASE_FOR_X11 (an internal definition) if xpkgwedge is
installed. Remove BUILD_DEPENDS on xpkgwedge (it would always have
been true anyway).

Add a new X11PREFIX definition, which reflects the PREFIX for X11
packages, and set it to LOCALBASE or X11BASE accordingly.

Remove all references to USE_LOCALBASE_FOR_X11 from the xpkgwedge
package.

In all, make the installation of xpkgwedge a tad less clumsy.