graphics/piglit: New port for the Mesa testsuite

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Summary

I just discovered that Piglit now supports a "make install" target, so there is no need to build and run Piglit from its source/build tree: we can distribute it.

There is no released source archive. Upstream sources are available in freedesktop's Git repository. I have a mirror on my GitHub with a patch for FreeBSD in the master-freebsd branch (I still need to upstream it). The port uses USE_GITHUB to fetch sources from this branch.

All tests, comprised of native binaries, Python scripts and data files, are all installed in $LOCALBASE/lib. I didn't try to split that in lib/libdata/libexec/share as it would require patching Piglit and I see no good reason for it. The plist is huge with more than 28000 files.

Test Plan

Once installed, you can run a sanity check:

• piglit run sanity /directory/for/results/storage
• piglit summary html /directory/for/summary/storage /directory/for/results/storage

An HTML summary is available in /directory/for/summary/storage/index.html.

Piglit usage is documented in /usr/local/share/doc/piglit/README.

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Same diff as the initial revision of this review, but uploaded manually instead of using arc(1).

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Add pkg-message to tell how to find usage instructions

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Fix build if libcaca is installed

A fix was committed to GitHub and the port uses this new revision.

Gbm and libcaca are now mandatory; likewise for OpenCL. No option is
available, everyone gets the same testsuite.

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Update to a snapshot of 2016-05-27

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Use a commit hash in $(GH_TAGNAME), not a branch name

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Use python2 in the piglit script

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Reduce pkg-plist size by generating part of it

The patch is from @kwm.

[kwm]