depends: boost: hard-code hidden symbol visibility

tl;dr: This solves boost visibility problems for default/release build configs
on non-Linux platforms.

When Bitcoin builds against boost's header-only classes, it ends up with
objects containing symbols that the upstream boost libs also have. Since
Bitcoin builds by default with hidden symbol visibility, it can end up trying
to link against a copy of the same symbols with default visibility.
This is not a problem on Linux because 3rd party static libs are un-exported
by default (--exclude-libs,ALL), but that is not available for MinGW and OSX.

Those platforms (and maybe others?) end up confused about which version to use.

The OSX linker spews hundreds of: "ld: warning: direct access in <foo> to
global weak symbol guard variable for <bar> means the weak symbol cannot be
overridden at runtime. This was likely caused by different translation units
being compiled with different visibility settings."

MinGW's linker complains similarly.

Since the default symbol visibility for Bitcoin is hidden and releases are
built that way as well, build Boost with hidden visibility. Linux builds Boost
this way also, but only for the sake of continuity.

This means that the linker confusion logic is reversed, so the problem will
will now be encountered if Bitcoin is built with --disable-reduce-exports, but
that's better than the current situation.
Rearrange initial config checks: AC, AM, compiler, libtool.
Removed some repeated and unused prog checks.
Reduce bitcoin-tx dependencies
Merge pull request #5213

0246ab6 depends: boost: hard-code hidden symbol visibility (Cory Fields)
Merge pull request #5223

bfe029f Reduce bitcoin-tx dependencies (Pieter Wuille)
Merge pull request #5214

eefb766 Rearrange initial config checks: AC, AM, compiler, libtool. Removed some repeated and unused prog checks. (kiwigb)
Delete src/secp256k1 before import
Squashed 'src/secp256k1/' content from commit ef6f677

git-subtree-dir: src/secp256k1
git-subtree-split: ef6f677679d7c2a5ba7d3ad85da0456a1d086a3e
Merge commit '19e3588f238bc216fceec8fd3899dac1d1465853' as 'src/secp256k1'
Always build and link libsecp256k1
Do not use EC code in global constructors
Use libsecp256k1 in key.cpp
Do optional signature-s negation inside the tests
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