WIP Build customization

[msarahan]

Implements #1142

Supersedes #966

Copying design goals from #1142:

Design goals:

• support config files at 3 levels, with lower levels overriding the upper ones:
  • included with conda-build
  • user's install-wide
  • per-package
  • document how older options, such as CONDA_PY and --python interact with/override config files
• Identify key confiugration types:
  • pinning versions (versions that must match between build and run)
  • variants of a particular feature (BLAS: MKL/OpenBLAS/ATLAS/etc.)
  • Specifying compiler / toolchain options
• Specify how to distinguish builds: how do different variations end up with different filenames?
  • Probably can't just have all variations contribute to build string directly.
  • Hash instead? Installation would need to access metadata, not rely on build string

[msarahan]

Merging. Outstanding issues:

• retry logic for compensating for conda's handling of the package cache fails builds on Travis. Working with conda team to come up with a proper fix for the package cache problems.
• Windows builds failing noarch tests due to conda packaging error. Being tracked at conda 4.3 missing resources folder - noarch packages broken conda#4715 - should be fixed in conda 4.3.14.

[jasongrout]

Wow, this is huge. Thanks! Is there a high-level summary added to the docs that I can read?

CC @aburgm, @SylvainCorlay

[jakirkham]

FYI apparently this added six as a requirement, but it doesn't appear in run currently.

[msarahan]

@jasongrout docs are at conda/conda-docs#414

@jakirkham thanks - fixed in #1766

[github-actions]

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