Issue #12641: Avoid passing "-mno-cygwin" to the mingw32 compiler, except when necessary.

Patch by Oscar Benjamin.

cygwinccompiler.py

@@ -48,7 +48,7 @@ cygwin in no-cygwin mode).
 import os
 import sys
 import copy
-from subprocess import Popen, PIPE
+from subprocess import Popen, PIPE, check_output
 import re
 
 from distutils.ccompiler import gen_preprocess_options, gen_lib_options
@@ -294,13 +294,18 @@ class Mingw32CCompiler(CygwinCCompiler):
         else:
             entry_point = ''
 
-        self.set_executables(compiler='gcc -mno-cygwin -O -Wall',
-                             compiler_so='gcc -mno-cygwin -mdll -O -Wall',
-                             compiler_cxx='g++ -mno-cygwin -O -Wall',
-                             linker_exe='gcc -mno-cygwin',
-                             linker_so='%s -mno-cygwin %s %s'
-                                        % (self.linker_dll, shared_option,
-                                           entry_point))
+        if self.gcc_version < '4' or is_cygwingcc():
+            no_cygwin = ' -mno-cygwin'
+        else:
+            no_cygwin = ''
+
+        self.set_executables(compiler='gcc%s -O -Wall' % no_cygwin,
+                             compiler_so='gcc%s -mdll -O -Wall' % no_cygwin,
+                             compiler_cxx='g++%s -O -Wall' % no_cygwin,
+                             linker_exe='gcc%s' % no_cygwin,
+                             linker_so='%s%s %s %s'
+                                    % (self.linker_dll, no_cygwin,
+                                       shared_option, entry_point))
         # Maybe we should also append -mthreads, but then the finished
         # dlls need another dll (mingwm10.dll see Mingw32 docs)
         # (-mthreads: Support thread-safe exception handling on `Mingw32')
@@ -393,3 +398,8 @@ def get_versions():
     """
     commands = ['gcc -dumpversion', 'ld -v', 'dllwrap --version']
     return tuple([_find_exe_version(cmd) for cmd in commands])
+
+def is_cygwingcc():
+    '''Try to determine if the gcc that would be used is from cygwin.'''
+    out_string = check_output(['gcc', '-dumpmachine'])
+    return out_string.strip().endswith(b'cygwin')