* Add 20181226 release of pgmath
* 20190329 release of pgmath requires match for CMPLX macro.
* Add llvm-flang package for Flang fork of LLVM
* Add new and old flang releases.
* Add cmake and python dependencies.
* Update dependencies on llvm-flang and pgmath.
* Fix cmake args and change spec to reflect llvm-flang package.
* change copyright date through 2020.
* Reference Flang package more explicitly.
* More robust support for python executable.
* import os no longer needed, picked up by flake8.
* Use built-in target spec. Variant and targets follow style in main LLVM package.
* Get rid of targets list and only support one target for now.
* Sparc does not appear to be supported in Flang.
* Raise InstallError if architecture not supported.
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