configure: add --with-amissl

configure: add --with-amissl

AmiSSL is an Amiga native library which provides a wrapper over OpenSSL.
It also requires all programs using it to use bsdsocket.library
directly, rather than accessing socket functions through clib, which
libcurl was not necessarily doing previously. Configure will now check
for the headers and ensure they are included if found.

Closes #3677