Because if you write your own Makefile instead of using automake, you'll get shot in the face by a distro package maintainer when you forget to obey DESTDIR, CC, CFLAGS, etc. like every packaging tool expects to use.
Because if you write your own Makefile instead of using automake, you'll get shot in the face by a distro package maintainer when you forget to obey DESTDIR, CC, CFLAGS, etc. like every packaging tool expects to use.