Merge #620: Install headers automatically

16e8615 Install headers automatically (Víctor Mayoral Vilches)

Pull request description:

  This fix install all the headers under include/ into
  /usr/local/include. The fix solves problems that arise
  when building libraries that depend on secp256k1 such
  as libbitcoin-system which require all the headers

Tree-SHA512: 8a5dc664b278e47340bf7478ad278306c44c4c8ad17a023b198c6a927c67c7a7a50100357388342129078afdf7606d2ed06579ce1fc14195fa974510b933021b