Add conan.tools.gnu.is_mingw() helper

[HYUEHFJKhfjklkej]

Summary

Adds a conan.tools.gnu.is_mingw(conanfile, build_context=False) helper.
Recipes commonly need to branch on a MinGW toolchain (Autotools instead
of NMake/MSBuild, MinGW-specific package_info, import-library naming
in package()); today they reimplement this check inline, often missing
edge cases.

This is a fresh take on the prior 1.x attempt (#12678), rewritten for
2.x with the review feedback applied:

• Early-return body for readability — the original review (@memsharded,
  @CJCombrink) flagged the dense boolean expression as the main blocker.
• No reliance on compiler.libcxx — it is removed in configure() of
  pure-C recipes, so it can't be used to disambiguate clang-mingw vs.
  clang-cl (@SpaceIm).
• compiler.runtime for clang disambiguation — clang-cl sets it,
  MinGW-Clang doesn't. Aligns with the official guidance in
  https://blog.conan.io/2022/10/13/Different-flavors-Clang-compiler-Windows.html
  (@mhthies).

Detection rules

A toolchain is identified as MinGW when:

Setting	Required value
os	"Windows"
os.subsystem	not "cygwin"
compiler	"gcc", or "clang" with compiler.runtime unset

Anything else (incl. msvc, clang-cl, Cygwin gcc, Linux/macOS gcc/clang)
returns False.

API

from conan.tools.gnu import is_mingw

class Recipe(ConanFile):
    def generate(self):
        if is_mingw(self):
            ...                    # MinGW-specific branch

build_context=True mirrors is_msvc() and consults settings_build
instead of host settings.

closes #15648

Test plan

Unit tests

New parametrized suite at test/unittests/tools/gnu/test_mingw.py covers:
gcc-mingw, MinGW-Clang, clang-cl, msvc, Cygwin gcc, Linux gcc/clang,
macOS apple-clang, build_context=True, and a C-only recipe with no
compiler.libcxx set.

Suite	Result
pytest test/unittests/tools/gnu/test_mingw.py	10 passed (Linux/macOS) + 10 passed (Windows Server 2022)
pytest test/unittests/tools/gnu/	164 passed
pytest test/unittests/tools/	710 passed, 8 skipped (pre-existing)

End-to-end on real Windows host

Verified via conan install on Windows Server 2022 Standard Evaluation
with four profiles, each driving a tiny recipe whose generate() calls
is_mingw(self):

Profile	Expected	Got
Windows + gcc-mingw + libstdc++11	True	✅ True
Windows + clang + compiler.runtime=dynamic (clang-cl)	False	✅ False
Windows + msvc 193	False	✅ False
Windows host → cross to Linux gcc	False	✅ False

Follow-up

Docs PR to follow at conan-io/docs once this lands.

[CLAassistant]

All committers have signed the CLA.

[memsharded]

This isn't necessarily true. The msys2 clang compiler is its own "environment", not a MinGW environment https://www.msys2.org/docs/environments/

the most common definition of is_mingw in ConanCenter recipes is:

 @property
    def _is_mingw(self):
        return self.settings.os == "Windows" and self.settings.compiler == "gcc"

So changing and doing it for clang too might be breaking some recipes or use cases?