Index: PCbuild/build_ssl.py
===================================================================
--- PCbuild/build_ssl.py	(revision 84935)
+++ PCbuild/build_ssl.py	(working copy)
@@ -23,7 +23,6 @@
 # python.exe build_ssl.py Release x64
 # python.exe build_ssl.py Release Win32
 
-from __future__ import with_statement
 import os, sys, re, shutil
 
 # Find all "foo.exe" files on the PATH.
@@ -122,7 +121,6 @@
     """
     if not os.path.isfile(makefile):
         return
-    fin = open(makefile)
     with open(makefile) as fin:
         lines = fin.readlines()
     with open(makefile, 'w') as fout:
@@ -148,6 +146,22 @@
     print(do_script)
     os.system(do_script)
 
+def cmp(f1, f2):
+    bufsize = 1024 * 8
+    with open(f1, 'rb') as fp1, open(f2, 'rb') as fp2:
+        while True:
+            b1 = fp1.read(bufsize)
+            b2 = fp2.read(bufsize)
+            if b1 != b2:
+                return False
+            if not b1:
+                return True
+
+def copy(src, dst):
+    if os.path.isfile(dst) and cmp(src, dst):
+        return
+    shutil.copy(src, dst)
+
 def main():
     build_all = "-a" in sys.argv
     if sys.argv[1] == "Release":
@@ -224,15 +238,17 @@
             if arch == "amd64":
                 create_makefile64(makefile, m32)
             fix_makefile(makefile)
-            shutil.copy2(r"crypto\buildinf.h", r"crypto\buildinf_%s.h" % arch)
-            shutil.copy2(r"crypto\opensslconf.h", r"crypto\opensslconf_%s.h" % arch)
+            copy(r"crypto\buildinf.h", r"crypto\buildinf_%s.h" % arch)
+            copy(r"crypto\opensslconf.h", r"crypto\opensslconf_%s.h" % arch)
 
         # If the assembler files don't exist in tmpXX, copy them there
-        if not os.path.exists("tmp"+dirsuffix):
-            os.mkdir("tmp"+dirsuffix)
-        for f in os.listdir("asm"+dirsuffix):
-            if not f.endswith(".asm"): continue
-            shutil.copy2(r"asm%s\%s" % (dirsuffix, f), "tmp"+dirsuffix)
+        if perl is None:
+            if not os.path.exists("tmp"+dirsuffix):
+                os.mkdir("tmp"+dirsuffix)
+            for f in os.listdir("asm"+dirsuffix):
+                if not f.endswith(".asm"): continue
+                if os.path.isfile(r"tmp%s\%s" % (dirsuffix, f)): continue
+                shutil.copy(r"asm%s\%s" % (dirsuffix, f), "tmp"+dirsuffix)
 
         # Now run make.
         if arch == "amd64":
@@ -241,8 +257,8 @@
                 print("ml64 assembler has failed.")
                 sys.exit(rc)
 
-        shutil.copy2(r"crypto\buildinf_%s.h" % arch, r"crypto\buildinf.h")
-        shutil.copy2(r"crypto\opensslconf_%s.h" % arch, r"crypto\opensslconf.h")
+        copy(r"crypto\buildinf_%s.h" % arch, r"crypto\buildinf.h")
+        copy(r"crypto\opensslconf_%s.h" % arch, r"crypto\opensslconf.h")
 
         #makeCommand = "nmake /nologo PERL=\"%s\" -f \"%s\"" %(perl, makefile)
         makeCommand = "nmake /nologo -f \"%s\"" % makefile