add CYTHON_CACHE_DIR env

resolution for the Cython cache dir:

1. CYTHON_CACHE_DIR env if defined
2. platform cache dir
   - (OS X): ~/Library/Caches/Cython if ~/Library/Caches exists
   - (posix not OS X): XDG_CACHE_HOME/cython if XDG_CACHE_HOME defined
3. ~/.cython/inline

Cython/Build/Inline.py

@@ -16,6 +16,7 @@ from Cython.Compiler.ParseTreeTransforms import CythonTransform, SkipDeclaration
 from Cython.Compiler.TreeFragment import parse_from_strings
 from Cython.Build.Dependencies import strip_string_literals, cythonize, cached_function
 from Cython.Compiler import Pipeline
+from Cython.Utils import get_cython_cache_dir
 import cython as cython_module
 
 # A utility function to convert user-supplied ASCII strings to unicode.
@@ -95,7 +96,7 @@ def safe_type(arg, context=None):
 
 def cython_inline(code,
                   get_type=unsafe_type,
-                  lib_dir=os.path.expanduser('~/.cython/inline'),
+                  lib_dir=get_cython_cache_dir(),
                   cython_include_dirs=['.'],
                   force=False,
                   quiet=False,

Cython/Utils.py

@@ -364,3 +364,32 @@ except NameError:
             if item:
                 return True
         return False
+
+@cached_function
+def get_cython_cache_dir():
+    """get the cython cache dir
+
+    Priority:
+
+    1. CYTHON_CACHE_DIR
+    2. (OS X): ~/Library/Caches/Cython
+       (posix not OS X): XDG_CACHE_HOME/cython if XDG_CACHE_HOME defined
+    3. ~/.cython/inline
+
+    """
+    if 'CYTHON_CACHE_DIR' in os.environ:
+        return os.environ['CYTHON_CACHE_DIR']
+
+    parent = None
+    if os.name == 'posix':
+        if sys.platform == 'darwin':
+            parent = os.path.expanduser('~/Library/Caches')
+        else:
+            # this could fallback on ~/.cache
+            parent = os.environ.get('XDG_CACHE_HOME')
+
+    if parent and os.path.isdir(parent):
+        return os.path.join(parent, 'cython')
+
+    # last fallback: ~/.cython/inline
+    return os.path.expanduser(os.path.join('~', '.cython', 'inline'))