571: Describe fallback platform checking using glibc version (#597)

pep-0571.rst

@@ -261,6 +261,31 @@ the ``_manylinux`` module described in PEP 513.  A platform is
 considered incompatible with ``manylinux2010`` if the attribute is
 ``False``.
 
+If the ``_manylinux`` module is not found, or it does not have the attribute
+``manylinux2010_compatible``, tools may fall back to checking for glibc. If the
+platform has glibc 2.12 or newer, it is assumed to be compatible unless the
+``_manylinux`` module says otherwise.
+
+Specifically, the algorithm we propose is::
+
+    def is_manylinux2010_compatible():
+        # Only Linux, and only x86-64 / i686
+        from distutils.util import get_platform
+        if get_platform() not in ["linux-x86_64", "linux-i686"]:
+            return False
+
+        # Check for presence of _manylinux module
+        try:
+            import _manylinux
+            return bool(_manylinux.manylinux2010_compatible)
+        except (ImportError, AttributeError):
+            # Fall through to heuristic check below
+            pass
+
+        # Check glibc version. CentOS 6 uses glibc 2.12.
+        # PEP 513 contains an implementation of this function.
+        return have_compatible_glibc(2, 12)
+
 
 Backwards compatibility with ``manylinux1`` wheels
 ==================================================