make CFG_OPTIMIZE_CXX an option

Author: nikomatsakis

configure

@@ -251,6 +251,7 @@ opt sharedstd 1 "build libstd as a shared library"
 opt valgrind 1 "run tests with valgrind"
 opt docs     1 "build documentation"
 opt optimize 1 "build optimized rust code"
+opt optimize-cxx 1 "build optimized C++ code"
 opt mingw-cross 0 "cross-compile for win32 using mingw"
 opt clang 0 "prefer gcc to clang for building the runtime"
 opt debug-llvm 0 "build LLVM in debug mode"