Makefile.am: make ctags ignore lwip-specific identifiers
Makefile.am: make ctags ignore lwip-specific identifiers

The Makefile uses ctags to build a list of global identifiers
that should be exported in a dynamic library. As ctags has a
very simplistic way of handling preprocessor conditionals, it
cannot detect symbols that are not present on platform POSIX.
In particular, the Solaris ld chokes on the following:

Undefined                       first referenced
 symbol                             in file
coap_pdu_from_pbuf                  ../.libs/libcoap-1.0.so
coap_packet_extract_pbuf            ../.libs/libcoap-1.0.so

The easiest way to fix this is to tell ctags that is should
ignore these identifiers with option -I.

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap,gebart/libcoap,gebart/libcoap,gebart/libcoap