Document conan.tools.gnu.is_mingw helper

reference/tools/gnu.rst

@@ -12,3 +12,4 @@ conan.tools.gnu
    gnu/makedeps
    gnu/pkgconfigdeps
    gnu/pkgconfig
+   gnu/mingw

reference/tools/gnu/mingw.rst

@@ -0,0 +1,39 @@
+.. _conan_tools_gnu_mingw:
+
+is_mingw
+========
+
+.. currentmodule:: conan.tools.gnu
+
+.. autofunction:: is_mingw
+
+The helper detects a MinGW toolchain from the recipe settings:
+
+* ``os`` is ``"Windows"``;
+* ``os.subsystem`` is **not** ``"cygwin"`` (Cygwin uses a POSIX layer
+  rather than a MinGW runtime);
+* ``compiler`` is ``"gcc"``, **or** it is ``"clang"`` with no
+  ``compiler.runtime`` set. ``clang-cl`` declares ``compiler.runtime``
+  and is therefore not classified as MinGW; see the
+  `Different flavors of Clang on Windows
+  <https://blog.conan.io/2022/10/13/Different-flavors-Clang-compiler-Windows.html>`_
+  blog post for the rationale.
+
+Pass ``build_context=True`` to inspect ``settings_build`` instead of the
+host settings (mirrors :ref:`is_msvc <conan_tools_microsoft_helpers>`).
+
+Example:
+
+.. code-block:: python
+
+    from conan import ConanFile
+    from conan.tools.gnu import is_mingw
+
+    class Pkg(ConanFile):
+        settings = "os", "arch", "compiler", "build_type"
+
+        def generate(self):
+            if is_mingw(self):
+                # MinGW-specific branch: Autotools-style build, MinGW
+                # import-library naming, etc.
+                ...