Warn when absolute paths are used with build_dir.

This closes #2323.

Cython/Build/Dependencies.py

@@ -964,7 +964,8 @@ def cythonize(module_list, exclude=None, nthreads=0, aliases=None, quiet=False,
 
                 # setup for out of place build directory if enabled
                 if build_dir:
-                    c_file = os.path.join(build_dir, c_file)
+                    if os.path.isabs(c_file):
+                      warnings.warn("build_dir has no effect for absolute source paths")
                     dir = os.path.dirname(c_file)
                     safe_makedirs_once(dir)