This has two main advantages: firstly, it actually makes the build slightly
faster and less memory intensive, because Idris doesn't fork a new
process and eat all the memory! Secondly, it means we can build a
distribution with the C file, for building on machines which don't have
Idris 1 available.
Also includes a script for building such a distribution.
I expect someone who is better a writing Makefiles than me can tidy this
up. There are almost certainly problems on Windows too - please help if
you can!
There are new Make targets 'all-fromc' and 'install-fromc' which don't
require building from Idris source, so that we can build with no Idris 1
available.
A small consequence: make install-exec no longer builds the idris2
executable, just installs it.
