-march=native assumes that we build the program on the
same system that we are going to run it, which can't be
assument to always be true. The compiler might emit
instructions which are not available on the target
system.

This option can be provided by the caller of the Makefile
if needed. E.g.:

  make CXXFLAGS="-mtune=native"

EXTRA_CXXFLAGS allow the caller to overwrite CXXFLAGS
defined in the makefile. E.g.:

    make EXTRA_CXXFLAGS="-O0"

This flags can be specified by the caller as needed. E.g:

    make EXTRA_CXXFLAGS="-g -O0"

Using $(MAKE) instead of "make", is special for gnu make. It
correctly processes Makeflags and parallel execution.