[configure.ac] Add the option of passing libnl path to configure script (sonic-net#1824)

**What I did**
Add the option of passing custmol built libnl path to configure script

**Why I did it**
MPLS feature in sonic-buildimage requires a libnl patch to be applied before building libnl. Since this build is not installed in usual locations (/usr/lib/..) LGTM analysis fails. This change gives the option of passing libnl library location to 'configure' script and generate libraries to be linked.

In case the options are not passed, the configure script defaults to earlier behavior where it checks for LIBNL in usual locations

Author: smaheshm

configure.ac

@@ -13,8 +13,6 @@ AC_HEADER_STDC
 AC_CHECK_LIB([hiredis], [redisConnect],,
     AC_MSG_ERROR([libhiredis is not installed.]))
 
-AC_CHECK_LIB([nl-genl-3], [genl_connect])
-
 AC_CHECK_LIB([team], [team_alloc],
     AM_CONDITIONAL(HAVE_LIBTEAM, true),
    [AC_MSG_WARN([libteam is not installed.])
@@ -46,7 +44,27 @@ AC_ARG_WITH(extra-lib,
                           prefix where extra libraries are installed],
 [AC_SUBST(LDFLAGS, "$LDFLAGS -L${withval}")])
 
-CFLAGS_COMMON="-std=c++14 -Wall -fPIC -Wno-write-strings -I/usr/include/libnl3 -I/usr/include/swss"
+AC_ARG_WITH(extra-usr-lib,
+[  --with-extra-usr-lib=DIR
+                          prefix where extra libraries are installed],
+[AC_SUBST(LDFLAGS, "$LDFLAGS -L${withval}")])
+
+AC_CHECK_LIB([nl-3], [nl_addr_alloc])
+AC_CHECK_LIB([nl-genl-3], [nl_socket_get_cb])
+AC_CHECK_LIB([nl-route-3], [rtnl_route_nh_get_encap_mpls_dst])
+AC_CHECK_LIB([nl-nf-3], [nfnl_connect])
+
+CFLAGS_COMMON="-std=c++14 -Wall -fPIC -Wno-write-strings -I/usr/include/swss"
+
+AC_ARG_WITH(libnl-3.0-inc,
+[  --with-libnl-3.0-inc=DIR
+                           prefix where libnl-3.0 includes are installed],
+[AC_SUBST(CPPFLAGS, "$CPPFLAGS -I${withval}")
+ AC_SUBST(LIBNL_INC_DIR, "${withval}")])
+
+if test "${with_libnl_3_0_inc+set}" != set; then
+    CFLAGS_COMMON+=" -I/usr/include/libnl3"
+fi
 
 CFLAGS_COMMON+=" -Werror"
 CFLAGS_COMMON+=" -Wno-reorder"