Issue 11715: Merge multiarch fix from 3.1 branch.

diff --cc setup.py
index 03b5eb75a01dc346072dfff34812891189688bbb,29a002fa3916c959d8e8f288ee19eb696a14b606..3b48b93a2319eee07bc4c562101a0c5af38e414a
--- 1/setup.py
--- 2/setup.py
+++ b/setup.py
@@@ -370,12 -339,31 +370,33 @@@ class PyBuildExt(build_ext)
                  return platform
          return sys.platform
  
+     def add_multiarch_paths(self):
+         # Debian/Ubuntu multiarch support.
+         # https://wiki.ubuntu.com/MultiarchSpec
+         tmpfile = os.path.join(self.build_temp, 'multiarch')
+         if not os.path.exists(self.build_temp):
+             os.makedirs(self.build_temp)
+         ret = os.system(
+             'dpkg-architecture -qDEB_HOST_MULTIARCH > %s 2> /dev/null' %
+             tmpfile)
+         try:
+             if ret >> 8 == 0:
+                 with open(tmpfile) as fp:
+                     multiarch_path_component = fp.readline().strip()
+                 add_dir_to_list(self.compiler.library_dirs,
+                                 '/usr/lib/' + multiarch_path_component)
+                 add_dir_to_list(self.compiler.include_dirs,
+                                 '/usr/include/' + multiarch_path_component)
+         finally:
+             os.unlink(tmpfile)
+ 
      def detect_modules(self):
 -        # Ensure that /usr/local is always used
 +        # Ensure that /usr/local is always used, but the local build
 +        # directories (i.e. '.' and 'Include') must be first.  See issue
 +        # 10520.
          add_dir_to_list(self.compiler.library_dirs, '/usr/local/lib')
          add_dir_to_list(self.compiler.include_dirs, '/usr/local/include')
+         self.add_multiarch_paths()
  
          # Add paths specified in the environment variables LDFLAGS and
          # CPPFLAGS for header and library files.