Add detect_gnu_libc() and detect_musl_libc() functions

[jwillikers]

Changelog: Feature: Add detect_libc to the detect_api to get the name and version of the C library.
Docs: conan-io/docs#3590

This PR adds a simple function to detect the version of glibc to the detect_api. This functionality is only intended for Linux systems using glibc. The function is detect_gnu_libc().
It simply runs ldd --version to detect the version of glibc. It is also possible to detect the version of the libc library by running the libc.so file. The location of this library varies much more than the path to ldd, so ldd --version is used instead. The path to ldd is passed as an argument, /usr/bin/ldd by default. This can modified to suite alternate sysroots.
It can even be used in cross-compilation scenarios where emulation is available to execute ldd.

The detect_gnu_libc function does a simple sanity check to verify that glibc is indeed being used. This avoids confusion on systems using an alternative libc implementation, i.e. musl.

I also added a function to detect the version of the musl libc library as well. This function is detect_musl_libc() and works the same way.

Addresses #3972, at least in part.

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[jwillikers]

Not sure the best way to add some automated unit tests for these functions, particularly since they are only applicable under Linux and need to run on a Linux system, unless I mock out ldd, which would probably be fine for unit tests.

[memsharded]

Thanks very much for your contribution @jwillikers

It is true that testing these is typically not easy.

I would test it very similarly to TestProfileDetectAPI in "functional" tests, without a strict validation of the specific return version, just checking that it doesn't crash and return a version is fine. For the musl that will not be in CI, checking that it returns None without crashing would be enough, as long as you test it locally and works for you.

[memsharded]

Functionally looks good, but it would be necessary to understand the usage patterns, which can be made explicit in a test.

Maybe it is better just a single function that returns libc, version = detect_api.detect_libc(), so libc gets the value glibc or musl?

[jwillikers]

That's definitely possible and seems useful. I think it might imply the scope applies to more than just Linux, however, since macOS, BSD*, and Windows are all going to have their own libc implementation. I guess the functionality could always be expanded in the future, but it is quite nice to be able to pass ldd as an argument, too. I'll work on getting the tests sorted out tomorrow.

[memsharded]

Yes, exactly, I think it can be expanded in the future in different forms like detect_api.detect_osx_libc(), via a new argument, or just by adding functionality if we are sure it will not break.

[jwillikers]

Alright, I've added a detect_libc function to be used instead of ones for the individual C libraries which returns a tuple of the name, i.e. gnu or musl, and the version.

I broke out the actual parsing logic into separate functions and added unit tests for this logic using the output of the ldd command.

I added a functional test for the detect_libc function.

[memsharded]

Looks good, just a minor detail about returning None instead of None, None

[memsharded]

Looks good, many thanks!
Merging this for next release 2.2.

I'd recommend documenting this as "experimental", we might discover cases we didn't think about it when other users use it.