It should be -O2 for the rts, but -O2 takes ages for idris2.c so should
be saved for releases (also OPT should be able to be edited easily for
e.g. adding profiling)
Also default to clang (perhaps it would be better if we could check
whether clang was available first, but it does a better job as the
default overall).
This is needed to get the time correctly on 32 bit systems (for example,
a Raspberry Pi!) otherwise it reports a negative number for the current
time...
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.
