bgpd: Warn user that bgp is setting maximum-paths larger than MULTIPATH_NUM

In cases where a distribution has compiled zebra with a MULTIPATH_NUM
that is less than what the user is setting the maximum-paths in bgp,
warn them that some of the nexthops will not make it into zebra and
by extension the kernel for routing.  The command is still accepted.

dell-s6000-02(config)# router bgp 100
dell-s6000-02(config-router)# maximum-paths 3
% Warning: maximum-paths set to 3 is greater than 2 that zebra is compiled to support
dell-s6000-02(config-router)# maximum-paths ibgp 44
% Warning: maximum-paths set to 44 is greater than 2 that zebra is compiled to support
dell-s6000-02(config-router)#

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>
diff --git a/bgpd/Makefile.am b/bgpd/Makefile.am
index 4659ac6..b2614f4 100644
--- a/bgpd/Makefile.am
+++ b/bgpd/Makefile.am
@@ -1,7 +1,7 @@
 ## Process this file with automake to produce Makefile.in.
 
 AM_CPPFLAGS = -I.. -I$(top_srcdir) -I$(top_srcdir)/lib -I$(top_builddir)/lib
-DEFS = @DEFS@ -DSYSCONFDIR=\"$(sysconfdir)/\"
+DEFS = @DEFS@ -DSYSCONFDIR=\"$(sysconfdir)/\" -DMULTIPATH_NUM=@MULTIPATH_NUM@
 INSTALL_SDATA=@INSTALL@ -m 600
 
 AM_CFLAGS = $(WERROR)