allow overriding optimization flags completely

If an optimization level ("-O<something>") is present in the argument
for --arch, do not prepend default optimization flags.

wscript

@@ -303,12 +303,21 @@ def set_compiler_flags (conf,opt):
     # prepend boiler plate optimization flags that work on all architectures
     #
 
+    optimization_flags[:0] = ["-pipe"]
+
+    # don't prepend optimization flags if "-O<something>" is present
+    prepend_opt_flags = True
+    for flag in optimization_flags:
+        if flag.startswith("-O"):
+            prepend_opt_flags = False
+            break
+
+    if prepend_opt_flags:
         optimization_flags[:0] = [
                 "-O3",
                 "-fomit-frame-pointer",
                 "-ffast-math",
-            "-fstrength-reduce",
-            "-pipe"
+                "-fstrength-reduce"
                 ]
 
     if opt.debug: