As of manual page INSTALL.md, it is stated that those variables can be
overridden by means of environmental variables.

export BINDIR="somedir"
export SBINDIR="somedir"
export LIBDIR="somedir"
export MANDIR="somedir"
export INCLUDEDIR="somedir"
export LIBEXECDIR="somedir"

make install

But those settings will not be honored, sticking to default Makefile values.
This patch fixes the issue.
Signed-off-by: Francesco Giancane <francescogiancane8@gmail.com>
Reviewed-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>