Use `-march=native` when building from install_easy.sh

install_easy.sh is used for local installation, which implies that if
it invokes make, the binaries are not being cross-compiled. That
allows us to pass `-march=native` to make sure the binaries are
optimized for the current CPU.

We prepend `-march=native` to CFLAGS to make sure that if a user sets
CFLAGS with a distinct `-march` option, ours will get overwritten.