make: Allow to install in custom dirs

There is no way to redefine install paths into
some custom ones (say plain /usr). Fix it by
using conditional inits.
Reported-by: Andrew Vagin <avagin@odin.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Andrew Vagin <avagin@odin.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

Makefile.inc

@@ -11,20 +11,20 @@ endif
 export E Q
 
 # Installation paths
-PREFIX		:= /usr/local
-SBINDIR		:= $(PREFIX)/sbin
-MANDIR		:= $(PREFIX)/share/man
-SYSTEMDUNITDIR	:= $(PREFIX)/lib/systemd/system/
-LOGROTATEDIR	:= $(PREFIX)/etc/logrotate.d/
-LIBDIR		:= $(PREFIX)/lib
+PREFIX		?= /usr/local
+SBINDIR		?= $(PREFIX)/sbin
+MANDIR		?= $(PREFIX)/share/man
+SYSTEMDUNITDIR	?= $(PREFIX)/lib/systemd/system/
+LOGROTATEDIR	?= $(PREFIX)/etc/logrotate.d/
+LIBDIR		?= $(PREFIX)/lib
 # 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)
 # For most other systems
 else ifeq "$(shell uname -m)" "x86_64"
-LIBDIR		:= $(PREFIX)/lib64
+LIBDIR		?= $(PREFIX)/lib64
 endif
 
-INCLUDEDIR	:= $(PREFIX)/include/criu
+INCLUDEDIR	?= $(PREFIX)/include/criu