Just built libassuan 2.1.0 on x86_64-sun-solaris2.10 with the no-cost Oracle
Workshop 12.2 compiler toolchain.

libassuan's configure checks for inet_pton, but it doesn't have any detection
code for OSes like Solaris, where many of the network-related functions are
split between libsocket and libnsl.

The attached patch seems to fix the issue for me.