Check for compiler for -march=native support

Check all compilers to see if they support the -march=native flags, rather
than assuming gcc 4.2 or later does. Tile GCC doesn't currently support it,
so not checking break Tile compiles.
Add a --set command line option to set/override a configuration value.
When setting a sequence value, check for an existing node.

This makes --set af-packet.0.threads=X do what you expect by
looking for the sequence node before setting.