diff -rupN Python-2.7.15/Lib/compileall.py Python-2.7.15-new/Lib/compileall.py
--- Python-2.7.15/Lib/compileall.py	2018-04-30 00:47:33.000000000 +0200
+++ Python-2.7.15-new/Lib/compileall.py	2018-05-02 17:23:32.438731542 +0200
@@ -42,6 +42,8 @@ def compile_dir(dir, maxlevels=10, ddir=
     success = 1
     for name in names:
         fullname = os.path.join(dir, name)
+        if sys.platform == "win32" and sys.version.find("GCC") >= 0:
+            fullname = fullname.replace('\\','/')
         if ddir is not None:
             dfile = os.path.join(ddir, name)
         else: