build: Rework how MULTIPATH_NUM is delivered to build

Changes made here:

1) MULTIPATH_NUM will never be 0.  If user specifies --enable-multipath=0
then this translates to MULTIPATH_NUM being set to 64 inside of the build
system.

2) Move MULTIPATH_NUM from a Makefile construct to a config.h construct.

3) Allowed MULTIPATH_NUM to be a number > 99 but < 1000

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>
diff --git a/bgpd/Makefile.am b/bgpd/Makefile.am
index 7da4dd8..d2775f3 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)/\" -DMULTIPATH_NUM=@MULTIPATH_NUM@
+DEFS = @DEFS@ -DSYSCONFDIR=\"$(sysconfdir)/\"
 INSTALL_SDATA=@INSTALL@ -m 600
 
 AM_CFLAGS = $(WERROR)