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Changes in XCode 11 Objective-C Runtime cause an error during build on
MacOS 10.15 Catalina.  Ported solution from bitcoin PR 16720.
Updated instruction on how to build berkeley DB for macOS 10.15
Catalina.

(Ported from bitcoin)

After this:

- `boost::optional` is no longer used directly (only through `Optional`
    which is an alias for it)
- `boost/optional.hpp` is only included in one place

cherry-picking commit 7cbfebbf3 from https://github.com/bitcoin/bitcoin

…inselection

On OS X, when searching Homebrew keg-only packages for BDB 4.8, if we find it,
use BDB_CPPFLAGS and BDB_LIBS instead of CFLAGS and LIBS for the result. This
is (1) more correct, and (2) necessary in order to give this location
priority over other directories in the include search path, which may include
system include directories with other versions of BDB.

This enables of the use of AI_* definitions in the Windows headers,
specifically AI_ADDRCONFIG, which fixes an issue with libevent and
ipv6 on Windows.

It also aligns with what we define in configure when building Core.

libevent uses getaddrinfo when available, and falls back to gethostbyname
Windows has both, but gethostbyname only supports IPv4
libevent fails to detect Windows's getaddrinfo due to not including the right headers
This patches libevent's configure script to check it correctly

configuration.

to compulsory C++17.

HexStr can be called with anything that bas `begin()` and `end()` functions,
so clean up the redundant calls.

Also moves $PTHREAD_CFLAGS to the CFLAGS.

Note that with this change we are no-longer including PTHREAD_* flags
when building libbitcoinconsensus.

Also note that we are including PTHREAD_LIBS in AM_PTHREAD_FLAGS

HexStr can be called with anything that bas `begin()` and `end()` functions,
so clean up the redundant calls.

Update macOS installation guide
Reference https://github.com/bitcoin/bitcoin
96124a204193ed114ca9594df7d5151206990e91 thru 48134a09adef3b5302cdd6e95500db404c9ac961

This has been around since the introduction of autotools. However at
this point I'm not sure we'd every want to suppress all warnings when
performing a build, and given that CXX FLAGS will have been overriden
when cross-compiling for Windows (using depends), this would rarely,
if-ever be used anyways.

From https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html:
-w

    Inhibit all warning messages.

Instruct the linker to set the major & minor subsystem versions in the PE
header to 6 & 1 (NT 6.1 which corresponds to Windows 7). Similar to
macOS, the binary will now refuse to run on unsupported versions of
Windows.

This change adds the correct suffix to debug mode .pc filenames for
MinGW and also to the Qt libraries listed in the `Requires` field.
The filename adjustment fixes the accidental overwriting of release
mode .pc files with the debug mode variant which required the wrong
variant of the libraries when `debug_and_release` is active.
Note that macOS also supports the `debug_and_release' configuration
but may use the regular library names together with DYLD_IMAGE_SUFFIX.
Creation of *_debug.pc files is turned off as they're identical to their
non-debug counterparts.
More info:
- QTBUG-4155
- Qt commit a0d8fb4ac3cb7bafdb39f340055eacee4f957513

This change adds to the BITCOIN_QT_CONFIGURE script ability to use
pkg-config for MinGW. All of the non-pkg-config paths are removed as
needless.
If depends is built with DEBUG=1 the configure script fails to pickup
Qt:
- for macOS host (similar, but not the same as issue 16391)
- for Windows host (regression)

QT_STATICPLUGIN is defined in BITCOIN_QT_CONFIGURE macro.

…support.

The Mingw-w64 5.0 (Ubuntu 18.04 Bionic) is used to build the Windows
binaries now.

It is safe to use pthread_setname_np since #17538 when the minimal glibc
version is set to 2.17.

common.vcxproj used for MSVC builds