[[maybe_unused]] (introduced in C++17) is standard C++.

https://clang.llvm.org/docs/AttributeReference.html#maybe-unused-unused
says that this is equivalent to the GNU unused attribute.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

[[nodiscard] (introduced in C++17) can be used instead.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

[[deprecated]] (introduced in C++14) may be used instead.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

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

It appears that [[fallthrough]] is valid in C++ but not in C. And
in some Clang versions (e.g. Clang 11 on macOS), Clang is pedantic
about this and considers it an error to use [[fallthrough]] in a
.c file such as src/util/format.c.

This changes makes gcc_fallthrough a no-op under Clang in C files.

Older clang versions don't support the GCC __attribute__ syntax.  For
those, don't use anything at all, and new clang versions shall use the
standard syntax.

Works around build failures with ccache which may feed processed code
to GCC, which doesn't have the "fall through" code comments.

Meanwhile, GCC 5 fails to compile MPD due to incomplete C++14
implementation.

Supporting GCC 4.9 is becoming cumbersome as we refactor more code to
C++14.

This shouldn't be a problem for anybody, because Debian Stable
(Stretch) has version 6.3, and Ubuntu LTS (Xenial) has version 5.3.

Check {GCC,CLANG}_VERSION==0 or >0 instead of using defined(), which
may render undefined behavior.

GCC_VERSION>0 implies defined(__GNUC__).

The ScopeExit library uses C++11 initializers, which gcc 4.6 does not
support.  Let's kill support for this ancient incomplete C++11
compiler, nobody should be using it anymore.

Copy the according file from another project (i.e. XCSoar).  This will
allow copying more code more easily.