feat: add build_triplet on conf

[ErniGH]

Changelog: Feature: Add tools.gnu:build_triplet to conf.
Docs: Omit

Close: #14962

[juansblanco]

Looks good!

[jcar87]

LGTM!

[jcar87]

For context:
for #14962 and #14947

From the documentation:
https://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/autoconf-2.72/html_node/Specifying-Target-Triplets.html#Specifying-Names

• When os and arch are both the same - Conan does not pass neither --host nor --build in Autotools toolchain
• When cross building (os/arch are different for build and host), Conan will autoguess some values for both --host and --build, but these are based on the most "default" cases and may not cover specifics. I can't think of a specific where one may only one to override the guessed build triple - so long as it differs from the host one, autotools will be in cross-compilation mode anyway - but it's good to give the option in case Conan's guess is not the most accurate.
• There can be cases where we have "incompatible" Linux flavour were os and arch are the same. This could be modelled by a separate setting (e.g. a subsetting in os) or not modelled at all (not recommended!).

@franramirez688, for the third case - I think there may be a case for replicating what we did in https://github.com/conan-io/conan/pull/15616/files here - in case anyone wants to "force" autotools into cross-building mode (this is telling autotools: do not try to run the executables you are building for the host platform) - that is, force Conan to pass --build and --host even when os and arch are the same. It would be around this line https://github.com/ErniGH/conan/blob/35b9792a257c5b4701038d6600ea9fb66d39bc4d/conan/tools/gnu/autotoolstoolchain.py#L71

Arguably the same goes for CMake - CMake assumes cross-compilation (or at least, CMAKE_CROSSCOMPILING evaluates to true) when CMAKE_SYSTEM_NAME is manually defined. We already have this as a conf, so technically it's already possible without touching recipes - but we may want to listen to tools.build.cross_building:cross_build and "force" it as well (I saw a case recently in an issue :D ) Just a thought!

[franramirez688]

@franramirez688, for the third case - I think there may be a case for replicating what we did in https://github.com/conan-io/conan/pull/15616/files here - in case anyone wants to "force" autotools into cross-building mode (this is telling autotools: do not try to run the executables you are building for the host platform) - that is, force Conan to pass --build and --host even when os and arch are the same. It would be around this line https://github.com/ErniGH/conan/blob/35b9792a257c5b4701038d6600ea9fb66d39bc4d/conan/tools/gnu/autotoolstoolchain.py#L71

@jcar87 I don't understand this comment correctly. That PR introduced a new conf in the conan.tools.build.cross_building function, i.e., available for all the toolchains, so they could use it right now. Did you mean to perform new tests using that conf through those toolchains?