Don't override CXXFLAGS optimisation levels.

Index: configure.ac
--- configure.ac.orig
+++ configure.ac
@@ -46,7 +46,6 @@ AC_C_BIGENDIAN
 
 # -- A R G U M E N T S ----------------------------------------
 
-AC_ARG_OPTIMIZATION
 AC_ARG_DEBUG
 AC_ARG_WARNINGS
 AC_ARG_PROFILING