Merge pull request #5220

f9e40fb revert part of 9eda1620b (Cory Fields)
4300876 build: secp256k1 as a subdir, since it's required (Cory Fields)
0dc8613 build: fixup configure to not export anything (Cory Fields)
54566de depends: quit exporting in config.site (Cory Fields)
ff1e5ba depends: add gmp package (Cory Fields)
cf61b54 Don't use bashisms in configure (Pieter Wuille)
dffb8f8 Use libsecp256k1 in key.cpp (Pieter Wuille)
4c97c64 Do not use EC code in global constructors (Pieter Wuille)
07a9901 Always build and link libsecp256k1 (Pieter Wuille)
d48555b Squashed 'src/secp256k1/' content from commit ad2028f (Pieter Wuille)
7a7e109 Delete src/secp256k1 before subtree import (Pieter Wuille)