Encode relative path to working tree in code paths when possible.

This makes the build reproducible irregardless of the build path. Fixes: #1565

Cython/Compiler/ExprNodes.py

@@ -9040,7 +9040,15 @@ class CodeObjectNode(ExprNode):
         func_name = code.get_py_string_const(
             func.name, identifier=True, is_str=False, unicode_value=func.name)
         # FIXME: better way to get the module file path at module init time? Encoding to use?
-        file_path = StringEncoding.bytes_literal(func.pos[0].get_filenametable_entry().encode('utf8'), 'utf8')
+        file_abspath = func.pos[0].get_filenametable_entry()
+        # Prefer relative paths to current directory (which is most likely the project root)
+        # over absolute paths.
+        workdir = os.getcwd() + os.sep
+        if file_abspath.startswith(workdir):
+            file_path = file_abspath[len(workdir):]
+        else:
+            file_path = file_abspath
+        file_path = StringEncoding.bytes_literal(file_path.encode('utf8'), 'utf8')
         file_path_const = code.get_py_string_const(file_path, identifier=False, is_str=True)
 
         flags = []