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.
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
the same source directory. The additional possibility just complicates
build system and makes future changes harder than necessary.