Support for PGI compilers

[nemequ]

They have a community edition now: http://www.pgroup.com/products/community.htm

• Build system — 9c35e23 did some, but…
  • -fno-builtin is currently being applied for non-MSVC compilers, but PGI doesn't support it
  • Figure out which flags to pass for optimization and (hopefully!) helpful warnings.
• Core
  • PGI doesn't support TLS (!). I think they do in C++, but not C. There is some code (in squash-splice.c) which needs to be ported to either the POSIX API or, better yet, removed in favor of something which doesn't require any TLS at all.
• Dependencies
  • Hedley ­— PGI is emitting some errors whenever I use conformant array parameters. C99-compliant compilers should always work, but if there is a bug in PGI we should work around it with another case for HEDLEY_ARRAY_PARAM.
  • Brotli
  • Gipfeli
  • libdeflate
• Travis — should be a lot easier than ICC; just need to wget the tarball, extract the installer, run it in silent mode (by setting a few environment variables), then set a few environment variables.