Make the Makefile variables externally configurable.

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>

Makefile.install

 #
 # Installation paths.
 PREFIX		?= /usr/local
-BINDIR		:= $(PREFIX)/bin
-SBINDIR		:= $(PREFIX)/sbin
-MANDIR		:= $(PREFIX)/share/man
-LIBDIR		:= $(PREFIX)/lib
-INCLUDEDIR	:= $(PREFIX)/include
-LIBEXECDIR	:= $(PREFIX)/libexec
+BINDIR		?= $(PREFIX)/bin
+SBINDIR		?= $(PREFIX)/sbin
+MANDIR		?= $(PREFIX)/share/man
+INCLUDEDIR	?= $(PREFIX)/include
+LIBEXECDIR	?= $(PREFIX)/libexec
 RUNDIR		?= /run
 
 #
 # For recent Debian/Ubuntu with multiarch support.
 DEB_HOST_MULTIARCH := $(shell dpkg-architecture -qDEB_HOST_MULTIARCH 2>/dev/null)
 ifneq "$(DEB_HOST_MULTIARCH)" ""
-        LIBDIR			:= $(PREFIX)/lib/$(DEB_HOST_MULTIARCH)
+        LIBDIR			?= $(PREFIX)/lib/$(DEB_HOST_MULTIARCH)
 else
         #
         # For most other systems
         ifeq "$(shell uname -m)" "x86_64"
-                LIBDIR		:= $(PREFIX)/lib64
+                LIBDIR		?= $(PREFIX)/lib64
         endif
 endif
 
+#
+# LIBDIR falls back to the standard path.
+LIBDIR ?= $(PREFIX)/lib
+
 export PREFIX BINDIR SBINDIR MANDIR RUNDIR
 export LIBDIR INCLUDEDIR LIBEXECDIR