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.

install_easy.sh

@@ -70,7 +70,7 @@ check_bins()
 	elif [ -f "$EXEDIR/Makefile" ] && exists make; then
 		echo trying to compile
 		[ "$SYSTEM" = "macos" ] && make_target=mac
-		make -C "$EXEDIR" $make_target || {
+		CFLAGS="-march=native ${CFLAGS}" make -C "$EXEDIR" $make_target || {
 			echo could not compile
 			make -C "$EXEDIR" clean
 			exitp 8