Add conan.tools.gnu.is_mingw() in conan API

[SpaceIm]

Changelog: (Feature): Add conan.tools.gnu.is_mingw() function
Docs: conan-io/docs#2849

Partially implement features requested in #12336 (comment). Such function is useful in recipes to know whether a user is using a canonical conan profile describing a MinGW toolchain, since there might be specific branching in a recipe based on this toolchain (using Autotools instead of NMake or MSBuild, deduce information for package info depending on logic in underlying build files etc).

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[SpaceIm]

it's worth noting that I've prefered this condition over conanfile.settings.get_safe("compiler.libcxx") because libcxx is removed in configure() of pure C recipes, so it's not a reliable compiler attribute in order to disambiguate between clang mingw and llvm-clang/clang-cl.

(but honestly if there was an extra attribute in conan settings of clang compiler in order to set windows flavor, it would be far easier)

[CLAassistant]

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[CJCombrink]

Also adding my comment here[1]:

This feels like a lot of "work" that will result in false in probably most cases (I don't have numbers but I assume mingw has quite a bit lower market share compared to gcc and msvc")
I guess it would still be better to get the short circuit behaviour compared to the old code where it returns false as soon as some condition is reached that makes it false.

I would propose something like early return:

def is_mingw(conanfile):
    if conanfile.settings.get_safe("os") != "Windows":
        return False
    if conanfile.settings.get_safe("os.subsystem") == "cygwin":
        return False
    host_compiler = conanfile.settings.get_safe("compiler")
    if host_compiler == "gcc":
        return True
    if host_compiler ==  "clang" and not conanfile.settings.get_safe("compiler.runtime"):
        return True
    return False

PS: From trying to map the current code to my snippet I can only conclude that the current code is very hard to read and understand (if my mapping is not correct it proves that the proposed code is perhaps overly complex).

[memsharded]

I agree with this, part this PR never got prioritized is because the logic reads too compact, and it also introduces new aspects that are not common practice in ConanCenter recipes, in which the majority just check Windows && gcc => MinGW

[memsharded]

One of the unknowns of this PR is that it introduced non-established practice in ConanCenter, in which all recipes just check for Windows&gcc, but things like cygwin or clang are not even considering.

As Conan 2.0 is already 1 year old, I am closing this PR for 1.X releases, the current workaround in recipes is enough, and I am moving it as a new ticket for 2.X releases, in order to be able to further improve recipes in the future

[memsharded]

I agree with this, part this PR never got prioritized is because the logic reads too compact, and it also introduces new aspects that are not common practice in ConanCenter recipes, in which the majority just check Windows && gcc => MinGW