Make ldconfig configurable

In a cross-compilation set up, running /sbin/ldconfig is useless, so
we make the path to ldconfig configurable through the environment/make
command line. This allows to pass LDCONFIG=/bin/true when doing
cross-compilation.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: b/make/Makefile.linux
===================================================================
--- a/make/Makefile.linux
+++ b/make/Makefile.linux
@@ -3,6 +3,7 @@
 #
 
 ARCH := $(shell uname -m)
+LDCONFIG ?= /sbin/ldconfig
 DESTDIR ?=
 LIBDIR =	$(DESTDIR)$(shell if [ "$(ARCH)" = "x86_64" -a -d "/usr/lib64" ]; then echo "/usr/lib64"; else echo "/usr/lib"; fi)
 
@@ -14,7 +15,7 @@
 
 PLUGIN_FULLNAME ?= $(PLUGIN_NAME).so.$(PLUGIN_VER)
 INSTALL_LIB =	install -D -m 755 $(PLUGIN_FULLNAME) $(LIBDIR)/$(PLUGIN_FULLNAME); \
-		/sbin/ldconfig -n $(LIBDIR)
+		$(LDCONFIG) -n $(LIBDIR)
 
 ifdef OLSRD_PLUGIN
 GENERATE_PIC = true