Add additional information on requirements in INSTALL.md

[keram]

and improve formatting in terminal.

Why:
This may help people prevent or resolve following issues
when running make bootstrap SCHEME=chez

• missing clang

make: clang: Command not found
make[1]: Entering directory 'support/c'
make[1]: clang: Command not found

• missing Chez thread support

Compiling idris2_app/idris2-boot.ss with output to idris2_app/idris2-boot.so
Exception: attempt to reference unbound identifier
make-thread-parameter at line 171, char 30 of idris2_app/idris2-boot.ss

$ less INSTALL.md before:

$ less INSTALL.md after:

[melted]

We can remove CC := clang, the problem that was fixed by that is gone.

[keram]

We can remove CC := clang, the problem that was fixed by that is gone.

You mean remove that from config.mk right?
You also mean that compiling using gcc is now as fast or faster than when using clang?
There used to be comment saying that clang was much faster so it make sense to use it

Idris2/config.mk

Line 5 in 1212e56

# clang compiles the output much faster than gcc!

I have noticed you removed that comment, Are there any benchmarks?

[ziman]

There's barely any C left in Idris 2 so the compiler doesn't really matter anymore.

[melted]

It used to be that Idris 2 was compiled by Idris 1 to a huge C file that clang handled much better. Now there's just a small support lib that is compiled in seconds by any compiler.

[keram]

Removed the mention of clang and also created the followup PR to remove it from config.mk as suggested. #183
Here I also replaced my original note about Chez

If you build your Chez Scheme from source please ensure it is build with 
[native thread support](https://github.com/cisco/ChezScheme/blob/master/BUILDING#L89-L91).

with similar note from tutorial/starting.rst about which I did not know when creating this PR.
@ziman Matus feeling brave to help get rid of those warnings ? 😉

[edwinb]

I'm not sure why I haven't merged this already, sorry... thanks for the updates, this question has come up a couple of times.