Makefile: introduce PREFIX and DESTDIR variables.
Makefile: introduce PREFIX and DESTDIR variables.

  PREFIX is used to set the common directory prefix for SYS*PATH
  variables.

  DESTDIR is for staged installs.

    https://www.gnu.org/prep/standards/html_node/DESTDIR.html

SHTOOLS/SHTOOLS,SHTOOLS/SHTOOLS,MarkWieczorek/SHTOOLS,MMesch/SHTOOLS,ioshchepkov/SHTOOLS,MarkWieczorek/SHTOOLS