This allows automatic optional detection of Sphinx.  This will be
useful when we start building the manpages with Sphinx, which many
users may want to have.

Move build rules from src/output/plugins/meson.build

Since Meson 0.51, there are special build options for "native:true"
builds, prefixed with "build.".  This change breaks cross builds
because `GenParseName.cxx` is no longer built with `-std=c++17`.

This patch adds defaults for "build.c_std" and "build.cpp_std".

Closes https://github.com/MusicPlayerDaemon/MPD/issues/890

For subprojects.

Enables `-Wpedantic`.

This branch isn't yet ready for level 3 (`-Wpedantic`) due to several
C++ violations (e.g. variable length arrays).  These are already
cleaned up in the master branch (0.22).

Work around Meson bug https://github.com/mesonbuild/meson/issues/3672

Commit 60f957ed broken the GCC 7 build, but instead of working
around missing C++17 features in old compilers, let's update the
compiler version requirements.

This commit raises the clang requirement to version 5 because this is
the first version to support `constexpr` lambdas, to be used to
`Dsd2Pcm.cxx`.

The event library uses various libnet.a classes,
e.g. SocketDescriptor.

Switch to std:: functions. Changed the meson check. Works locally.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Signed-off-by: Rosen Penev <rosenp@gmail.com>

Prevents compiling those sources many times, once for each debug
program using it.

By accident, this was only enabled for C.

Fixes another build failure.

Prepare for using other crypto libraries, e.g. FFmpeg's libavutil.

Time to move on, two years after 2017.