* Promote to Autotools for simplicity; broaden Intel CPU opt targeting from SSE4.2 to AVX2
* make Intel CPU opt targeting same as in ../libxc
* flake8 W291
* use canonical means to pass (ahem) the test phase, h/t @adamjstewart
* revert f25d598 (unrelated merge)
* re-merge
* Initial work on flag trapping using functions called <flag>_handler and default_flag_handler
* Update packages so they do not obliterate flags
* Added append to EnvironmentModifications class
* changed EnvironmentModifications to have append_flags method
* changed flag_val to be a tuple
* Increased test coverage
* added documentation of flag handling
* libint: updated version, ported to AutotoolsPackage
* elpa: updated version, ported to AutotoolsPackage
* wannier90: updated version, ported to MakefilePackage
* pexsi: updated version, ported to MakefilePackage
* cp2k: updated version, removed wannier 90 from dependencies if cp2k@4.1
* plumed: updated version
Modifications:
* modules has been turned into a single variant, as different versions
have different modules available
* added dictionaries for patches
* netlib-scalapack: ported to CMakePackage
* Add libint package
* Add Intel optimization flags recommended by CP2K
* Add new version and Intel compiler optimization flags for libxc
* Add older version of libint
* Libint depends on GMP C++ library
