Modify compile flags handling in Makefile for Debian packaging This is to allow CFLAGS set by dpkg-buildflags from the environment so that hardening flags and DEB_BUILD_OPTIONS=noopt would work. . Also disable -march=native which causes FTBFS on many platforms and is inappropriate for a Debian package. See a similar bug https://bugs.debian.org/761912 for an explanation. (Note: Upstream commented out "CFLAGS += -march=native" in commit 0ead90d5d9ab861acd40a05490ed8e7936ac1fa5 on 2019-11-12.)
