Import libsecp256k1 commit d0b33489f282ae3675cb4fc5ae4c2c9381df4103
Support for libsecp256k1
gitian support for secp256k1 linux builds
secp256k1 in makefile.osx
gitignore secp256k1 files
secp256k1 win32 build for 0.8.x

FIXME: secp256k1 build hacks

Conflicts:
	contrib/gitian-descriptors/gitian-win32.yml
Allow gmp to be built static for gitian builds