So long, autotools!  This is my last MPD related project to migrate
away from it.  It has its strengths, but also very obvious weaknesses
and weirdnesses.  Today, many of its quirks are not needed anymore,
and are cumbersome and slow.  Now welcome our new Meson overlords!

GCC 6.3.0 emits bogus warnings like this:

 In file included from src/lib/dbus/UDisks2.cxx:22:0:
 src/lib/dbus/ReadIter.hxx: In instantiation of 'ODBus::ReadMessageIter::ForEachProperty(F&&)::<lambda(auto:2&&)> [with auto:2 = ODBus::ReadMessageIter; F = std::_Bind<void (*(std::reference_wrapper<UDisks2::Object>, std::_Placeholder<1>, std::_Placeholder<2>))(UDisks2::Object&, const char*, ODBus::ReadMessageIter&&) noexcept>]':
 src/lib/dbus/ReadIter.hxx:102:6:   required from 'ODBus::ReadMessageIter::ForEachRecurse(int, F&&)::<lambda(auto:1&&)> [with auto:1 = ODBus::ReadMessageIter&; F = ODBus::ReadMessageIter::ForEachProperty(F&&) [with F = std::_Bind<void (*(std::reference_wrapper<UDisks2::Object>, std::_Placeholder<1>, std::_Placeholder<2>))(UDisks2::Object&, const char*, ODBus::ReadMessageIter&&) noexcept>]::<lambda(auto:2&&)>]'
 src/lib/dbus/ReadIter.hxx:92:5:   required from 'void ODBus::ReadMessageIter::ForEach(int, F&&) [with F = ODBus::ReadMessageIter::ForEachRecurse(int, F&&) [with F = ODBus::ReadMessageIter::ForEachProperty(F&&) [with F = std::_Bind<void (*(std::reference_wrapper<UDisks2::Object>, std::_Placeholder<1>, std::_Placeholder<2>))(UDisks2::Object&, const char*, ODBus::ReadMessageIter&&) noexcept>]::<lambda(auto:2&&)>]::<lambda(auto:1&&)>]'
 src/lib/dbus/ReadIter.hxx:101:3:   required from 'void ODBus::ReadMessageIter::ForEachRecurse(int, F&&) [with F = ODBus::ReadMessageIter::ForEachProperty(F&&) [with F = std::_Bind<void (*(std::reference_wrapper<UDisks2::Object>, std::_Placeholder<1>, std::_Placeholder<2>))(UDisks2::Object&, const char*, ODBus::ReadMessageIter&&) noexcept>]::<lambda(auto:2&&)>]'
 src/lib/dbus/ReadIter.hxx:114:3:   required from 'void ODBus::ReadMessageIter::ForEachProperty(F&&) [with F = std::_Bind<void (*(std::reference_wrapper<UDisks2::Object>, std::_Placeholder<1>, std::_Placeholder<2>))(UDisks2::Object&, const char*, ODBus::ReadMessageIter&&) noexcept>]'
 src/lib/dbus/UDisks2.cxx:71:30:   required from here
 src/lib/dbus/ReadIter.hxx:114:53: error: declaration of 'ODBus::ReadMessageIter&& i' shadows a parameter [-Werror=shadow]
    ForEachRecurse(DBUS_TYPE_DICT_ENTRY, [&f](auto &&i){
                                                      ^
 src/lib/dbus/ReadIter.hxx:101:33: note: shadowed declaration is here
    ForEach(arg_type, [&f](auto &&i){
                                  ^

Documentation will follow soon.

Avoid interfering with libraries also using the shared connection.

From https://github.com/CM4all/libcommon/commit/a7cef404c1f7eedff1a629da48f77491218ee281

To be used by upcoming code.