Makefile overhaul, moving PREFIX to CROSS_COMPILE, providing PREFIX and DESTDIR

[GitMensch]

• CROSS_COMPILE will be used as a prefix to the executed toolss
• PREFIX may default to /usr/local if not explicit set
• DESTDIR will only be used in the (un)install targets and as expected prefix the PREFIX

@Bill-Gray maybe that's something possible for upcoming 4.5.2 (or do you see the changes already done since 4.5.1 and/or breaking existing PREFIX usages during build as needing a new minor version)?