examples/lwip/Makefile: fixed lwip build flags
examples/lwip/Makefile: fixed lwip build flags

Since configure does not set WITH_LWIP, we need to tell the compiler.
This change also replaces the deprecated -I. -I- by -iquote.

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