Drop support for multiple configurations

Drop support for multiple configurations

Currently configure has the flag --enable-config=CONF which allows to
have multiple configurations in the same directory and choose from them
using `make conf=CONF'. This might have been useful in the last century
(yes, the flag was really added in 1998 with LilyPond 1.0.1!) ...
Nowadays the solution is to just have many build directories, pointing to
the same source directory. The additional possibility just complicates the
build system and makes future changes harder than necessary.

[lemzwerg, 2020-02-12 16:39:25]

LGTM, thanks!

[Carl, 2020-02-12 16:50:31]

I really like the work.  I just wonder if we should eliminate the instructions
on how to achieve the objective.

https://codereview.appspot.com/581630043/diff/563520043/Documentation/include...
File Documentation/included/compile.itexi (left):

https://codereview.appspot.com/581630043/diff/563520043/Documentation/include...
Documentation/included/compile.itexi:895: @node Compiling for multiple platforms
I wonder if we should keep the heading "Compiling for multiple platforms", and
just have it say "Use a separate build directory for each different platform,
and run configure in each build directory."  Perhaps even use the same example,
and show how to handle this situation.

[hanwenn, 2020-02-12 20:43:51]

Oh no! I was using this just this week for GUILE2 vs GUILE1.8


(but OK to remove, I agree it is too baroque).

[hanwenn, 2020-02-12 20:45:31]

https://codereview.appspot.com/581630043/diff/563520043/Documentation/include...
File Documentation/included/compile.itexi (left):

https://codereview.appspot.com/581630043/diff/563520043/Documentation/include...
Documentation/included/compile.itexi:895: @node Compiling for multiple platforms
On 2020/02/12 16:50:30, Carl wrote:
> I wonder if we should keep the heading "Compiling for multiple platforms", and
> just have it say "Use a separate build directory for each different platform,
> and run configure in each build directory."  Perhaps even use the same
example,
> and show how to handle this situation.

people that need to do this are probably expert enough to already know how to
achieve it.

[hahnjo, 2020-02-13 08:05:04]

On 2020/02/12 20:45:31, hanwenn wrote:
>
https://codereview.appspot.com/581630043/diff/563520043/Documentation/include...
> File Documentation/included/compile.itexi (left):
> 
>
https://codereview.appspot.com/581630043/diff/563520043/Documentation/include...
> Documentation/included/compile.itexi:895: @node Compiling for multiple
platforms
> On 2020/02/12 16:50:30, Carl wrote:
> > I wonder if we should keep the heading "Compiling for multiple platforms",
and
> > just have it say "Use a separate build directory for each different
platform,
> > and run configure in each build directory."  Perhaps even use the same
> example,
> > and show how to handle this situation.
> 
> people that need to do this are probably expert enough to already know how to
> achieve it.

Right, this is core functionality from Autoconf and described in their
documentation. Additionally I don't think "multiple platforms" is a thing
nowadays - there's x86_64 and that's mostly it what you compile on a single
system.