The latter is used by packages that build libraries from the Mesa
sources, while the former is only if the package builds from the Mesa
sources. Modify the various Mesa packages to include the proper
Makefile, and add some documentation to the Makefiles.
As a side effect, this suppresses the "duplicate target" warning in
graphics/glx-utils that was caused by having the same distfile listed
in DISTFILES more than once.
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