Do signature-s negation inside the tests

To avoid the need for libsecp256k1 to expose such functionality.
Squashed 'src/secp256k1/' content from commit ef6f677

git-subtree-dir: src/secp256k1
git-subtree-split: ef6f677679d7c2a5ba7d3ad85da0456a1d086a3e
Delete src/secp256k1 before import
Merge commit 'a4d66b1c24c601191bfb13c34bd759767c05b10b' as 'src/secp256k1'
Always build and link libsecp256k1
Do not use EC code in global constructors
Use libsecp256k1 in key.cpp
Don't use bashisms in configure
depends: add gmp package
depends: quit exporting in config.site
build: fixup configure to not export anything
build: secp256k1 as a subdir, since it's required
revert part of 9eda1620b

This probably would've broken native OSX builds