ASCEND is migrating to Github
use 'in' instead of 'has_key'
comment out the part that creates the NSIS installer, nsis.py doesn't work yet with Python3 apparently.
updated spec file
added to changelog
updates to the SCons build scripts to allow use of SCons 4.3 with Python 3.6.
corrections to allow compilation in Ubuntu 20.04
add checks for C header files. add flag WITH_GTK to suppress building of GUI.