Merge #928: Define SECP256K1_BUILD in secp256k1.c directly.

ae9e648526ceaf7cd97ba4dfe3c105db8e226c35 Define SECP256K1_BUILD in secp256k1.c directly. (Gregory Maxwell)

Pull request description:

  This avoids building without it and makes it safer to use a custom
   building environment.  Test harnesses need to #include secp256k1.c
   first now.

  Fixes #927

ACKs for top commit:
  sipa:
    utACK ae9e648526ceaf7cd97ba4dfe3c105db8e226c35
  real-or-random:
    ACK ae9e648526ceaf7cd97ba4dfe3c105db8e226c35

Tree-SHA512: 65ccc15c18f111ba926db1bb25f06c2beb2997c6f42c6d3ebc371ca84f4b5918379efd25c30556cedfd2e4275758bd79d733e80a11159c6ec013dd4707a683ad