bpo-21536: On Cygwin, C extensions must be linked with libpython (GH-13549)

It is also possible to link against a library or executable with a
statically linked libpython, but not both with the same DLL.  In fact
building a statically linked python is currently broken on Cygwin
for other (related) reasons.

The same problem applies to other POSIX-like layers over Windows
(MinGW, MSYS) but Python's build system does not seem to attempt
to support those platforms at the moment.

command/build_ext.py

@@ -714,20 +714,32 @@ class build_ext(Command):
                 # don't extend ext.libraries, it may be shared with other
                 # extensions, it is a reference to the original list
                 return ext.libraries + [pythonlib]
-        # On Android only the main executable and LD_PRELOADs are considered
-        # to be RTLD_GLOBAL, all the dependencies of the main executable
-        # remain RTLD_LOCAL and so the shared libraries must be linked with
-        # libpython when python is built with a shared python library (issue
-        # bpo-21536).
         else:
+            # On Android only the main executable and LD_PRELOADs are considered
+            # to be RTLD_GLOBAL, all the dependencies of the main executable
+            # remain RTLD_LOCAL and so the shared libraries must be linked with
+            # libpython when python is built with a shared python library (issue
+            # bpo-21536).
+            # On Cygwin (and if required, other POSIX-like platforms based on
+            # Windows like MinGW) it is simply necessary that all symbols in
+            # shared libraries are resolved at link time.
             from distutils.sysconfig import get_config_var
+            link_libpython = False
             if get_config_var('Py_ENABLE_SHARED'):
-                # Either a native build on an Android device or the
-                # cross-compilation of Python.
-                if (hasattr(sys, 'getandroidapilevel') or
-                        ('_PYTHON_HOST_PLATFORM' in os.environ and
-                         get_config_var('ANDROID_API_LEVEL') != 0)):
-                    ldversion = get_config_var('LDVERSION')
-                    return ext.libraries + ['python' + ldversion]
+                # A native build on an Android device or on Cygwin
+                if hasattr(sys, 'getandroidapilevel'):
+                    link_libpython = True
+                elif sys.platform == 'cygwin':
+                    link_libpython = True
+                elif '_PYTHON_HOST_PLATFORM' in os.environ:
+                    # We are cross-compiling for one of the relevant platforms
+                    if get_config_var('ANDROID_API_LEVEL') != 0:
+                        link_libpython = True
+                    elif get_config_var('MACHDEP') == 'cygwin':
+                        link_libpython = True
+
+            if link_libpython:
+                ldversion = get_config_var('LDVERSION')
+                return ext.libraries + ['python' + ldversion]
 
         return ext.libraries