make: defer prepending DESTDIR until install

The DESTDIR variable is often used in the context of cross
compilation to a target sysroot. The value of DESTDIR should
not be built in to the resulting binary. Instead, use PREFIX
to point to an alternative local directory.
make: use /usr/local as default PREFIX

This is to align more with Filesystem Hierarchy Standard, specifying that
/usr/local should be used "when installing software locally".
make: use weak assignment where relevant

This way you can override make's behavior with environment variables.
make: don't chown to root when installing

Either the installing user is root, and the owner would become root,
or the installing user isn't root and the operation would most likely
not be permitted.
make: add support for specifying LDFLAGS
Merge pull request #3 from olof/makefile-fixes

Some Makefile fixes