Allow to configure /var/run/netns directory

Currently NETNS_RUN_DIR is hardcoded and refers to /var/run/netns.
However, some systems (e.g. Android) doesn't have /var
which results in error attempts to create network namespaces on these
systems.  This change makes NETNS_RUN_DIR configurable at build time
by allowing to pass environment variable to make command.
Also, this change makes /etc/netns directory configurable through
NETNS_ETC_DIR environment variable.

For example: ./configure && NETNS_RUN_DIR=/mnt/vendor/netns make

Tested: verified that iproute2 with configuration mentioned above
creates namespaces in /mnt/vendor/netns

Signed-off-by: Pavel Maltsev <pavelm@google.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>

Makefile

@@ -16,6 +16,8 @@ PREFIX?=/usr
 LIBDIR?=$(PREFIX)/lib
 SBINDIR?=/sbin
 CONFDIR?=/etc/iproute2
+NETNS_RUN_DIR?=/var/run/netns
+NETNS_ETC_DIR?=/etc/netns
 DATADIR?=$(PREFIX)/share
 HDRDIR?=$(PREFIX)/include/iproute2
 DOCDIR?=$(DATADIR)/doc/iproute2
@@ -34,7 +36,9 @@ ifneq ($(SHARED_LIBS),y)
 DEFINES+= -DNO_SHARED_LIBS
 endif
 
-DEFINES+=-DCONFDIR=\"$(CONFDIR)\"
+DEFINES+=-DCONFDIR=\"$(CONFDIR)\" \
+         -DNETNS_RUN_DIR=\"$(NETNS_RUN_DIR)\" \
+         -DNETNS_ETC_DIR=\"$(NETNS_ETC_DIR)\"
 
 #options for decnet
 ADDLIB+=dnet_ntop.o dnet_pton.o

include/namespace.h

@@ -8,8 +8,13 @@
 #include <sys/syscall.h>
 #include <errno.h>
 
+#ifndef NETNS_RUN_DIR
 #define NETNS_RUN_DIR "/var/run/netns"
+#endif
+
+#ifndef NETNS_ETC_DIR
 #define NETNS_ETC_DIR "/etc/netns"
+#endif
 
 #ifndef CLONE_NEWNET
 #define CLONE_NEWNET 0x40000000	/* New network namespace (lo, device, names sockets, etc) */