add DESTDIR, MANDIR to install target
add DESTDIR, MANDIR to install target

DESTDIR to allow installing in chroot
MANDIR to allow specifying different dir for man pages

mimimitm/sslsplit,heavilessrose/sslsplit,mimimitm/sslsplit,DJMIN/sslsplit,DJMIN/sslsplit,heavilessrose/sslsplit