* respect system toolchain: gcc, pkg-config etc might be prefixed
* allow more control over passed flags, never overwriting CFLAGS etc
  entirely
* proper quoting: make does _not_ take care of quoting in make rules
* default "install" target installs into system plugin dir, additional
  "install-home" target installs into users home plugin dir
* simplify "clean" target and add "clean-all" target
* some minor changes to follow common Make style
* update .PHONY