Make this package xpkgwedge-safe by using the new X11PREFIX definition

Make this package xpkgwedge-safe by using the new X11PREFIX definition
in preference to X11BASE when trying to find the installed location of
X11 libraries, programs and headers (which will change if xpkgwedge is
installed).

[X11BASE reflects the location of the X11 libs, headers and programs.
X11PREFIX reflects the installed location of X11 packages.]