Use _REQUIRE_BUILTIN_* to specify that the headers/libraries of some parts
of X11 that are available from 3rd parties must be part of the standard X11
distribution or else their symlinks aren't installed as part of x11-links.
This is to allow x11-links to contain all of the parts of the standard X11
distribution.
The problem that prompted this is that newer releases of XFree86-4.x
contain not just new libraries but new inter-library dependencies. Some
of the X11 libraries present in XF86-4.x that were also in XF86-3.x now
depend on libXpm.so or libfreetype.so, which, from a buildlink
point-of-view, must also be found during the link phase when building
software.
