message: Add env variable for setup.py extra cmake args
  When running programs like cibuildwheel, this allows us to more
  easily append extra cmake arguments to the list via an environment
  variable. The cmake arguments are semicolon delimited.
  Example:
  EXTRA_CMAKE_ARGS=-DEIGEN3_INCLUDE_DIR=/c/eigen;-DBUILD_TYPE=Release
diff: +import os
  +
  from skbuild import setup
  +
  +def extra_cmake_args():
  + env = os.getenv('EXTRA_CMAKE_ARGS')
  + return env.split(';') if env else []
  +
  +
  +cmake_args = [
  + '-DUSE_SPGLIB:BOOL=FALSE',
  + '-DUSE_OPENGL:BOOL=FALSE',
  + '-DUSE_QT:BOOL=FALSE',
  + '-DUSE_MMTF:BOOL=FALSE',
  + '-DUSE_PYTHON:BOOL=TRUE',
  + '-DUSE_MOLEQUEUE:BOOL=FALSE',
  + '-DUSE_HDF5:BOOL=FALSE',
  + '-DUSE_LIBARCHIVE:BOOL=FALSE',
  + '-DUSE_LIBMSYM:BOOL=FALSE',
  +] + extra_cmake_args()
  +
  setup(
  name='avogadro',
  use_scm_version=True,
  @@ -24,16 +44,5 @@ setup(
  'Operating System :: MacOS'
  ],
  packages=['avogadro'],
  - cmake_args=[
  - '-DUSE_SPGLIB:BOOL=FALSE',
  - '-DUSE_OPENGL:BOOL=FALSE',
  - '-DUSE_QT:BOOL=FALSE',
  - '-DUSE_MMTF:BOOL=FALSE',
  - '-DUSE_PYTHON:BOOL=TRUE',
  - '-DUSE_MOLEQUEUE:BOOL=FALSE',
  - '-DUSE_HDF5:BOOL=FALSE',
  - '-DUSE_LIBARCHIVE:BOOL=FALSE',
  - '-DUSE_LIBMSYM:BOOL=FALSE'
  - ]
  + cmake_args=cmake_args,
  )
  -
  