Respect user optimization and debug flags, as in Gentoo's gn-gen patch.

--- a/build/gen.py
+++ b/build/gen.py
@@ -538,10 +538,6 @@
         cflags.extend(['-D_LIBCPP_DEBUG=1', '-D_GLIBCXX_DEBUG=1'])
     else:
       cflags.append('-DNDEBUG')
-      cflags.append('-O3')
-      if options.no_strip:
-        cflags.append('-g')
-      ldflags.append('-O3')
       # Use -fdata-sections and -ffunction-sections to place each function
       # or data item into its own section so --gc-sections can eliminate any
       # unused functions and data items.
@@ -589,7 +585,6 @@
         '-D_FILE_OFFSET_BITS=64',
         '-D__STDC_CONSTANT_MACROS', '-D__STDC_FORMAT_MACROS',
         '-pthread',
-        '-pipe',
         '-fno-exceptions',
         '-fno-rtti',
         '-fdiagnostics-color',