[docs] Add mini-"howto" using Qt Creator (osx)
[Qt, OSX] fix usage of osx 10.8+ user notification center

Currently Bitcoin-Qts support for OSX User Notification Center is broken. This pull will fix a known issue of non-official-apple-built apps having problems sending user notifications.
Squashed 'src/secp256k1/' content from commit ad2028f

git-subtree-dir: src/secp256k1
git-subtree-split: ad2028f9890ca40bdd32055aa0fe5c1c9af0e485
Delete src/secp256k1 before subtree import
Merge commit 'd48555b36ac512161b81f9b6bca7bea16a0cd806' 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
Merge pull request #5300

a7f2941 [Qt, OSX] fix usage of osx 10.8+ user notification center (Jonas Schnelli)
Merge pull request #5279

6c8c704 [docs] Add mini-"howto" using Qt Creator (osx) (Jonas Schnelli)
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)