make: Makefile variable 'PREFIX' should be configurable.

I tried to build and install the criu package from source. All went
well, unless for the install part.

In particular,
PREFIX=/
DESTDIR=~/build_criu

PREFIX=${PREFIX} DESTDIR=${DESTDIR} make install

resulted in installing criu packages in ~/build_criu/usr/local/,
ignoring PREFIX, even if this variable should be configurable as of
manual and GCS.
Reviewed-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Francesco Giancane <francescogiancane8@gmail.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

Makefile.install

 #
 # Installation paths.
-PREFIX		:= /usr/local
+PREFIX		?= /usr/local
 BINDIR		:= $(PREFIX)/bin
 SBINDIR		:= $(PREFIX)/sbin
 MANDIR		:= $(PREFIX)/share/man