autotools: autotools'ify libsecp256k1
add missing include
autotools: changes suggested by Pieter Wuille
autotools: No need to link against libssl
Merge pull request #14

6e7a6d4 autotools: No need to link against libssl (Cory Fields)
0e9baf6 autotools: changes suggested by Pieter Wuille (Cory Fields)
5a9989c add missing include (Cory Fields)
78cd96b autotools: autotools'ify libsecp256k1 (Cory Fields)

Signed-off-by: Pieter Wuille <pieterw@google.com>