pd-lua now uses the latest pd-lib-builder and includes lua as a
submodule, so building it becomes a lot easier. Most notably, Lua 5.4 is
linked statically from lua/onelua.c, so Lua isn't needed as a build or
runtime dependency any more and the resulting external will use exactly
the same Lua version across different platforms.

On Apple Silicon, Homebrew is installed in a different location. Thus
in the externals Makefile and the embed-MacOSX-dependencies.sh script,
we now check the HOMEBREW_PREFIX environment variable to determine
whether Homebrew is installed and, if so, where it is.

Also added missing include and library paths in some externals so that
they actually find the Homebrew header files and libraries.

Specifically, we disable the branches which attempt to detect iOS
cross compilation by checking for a Darwin system with arm cpu
architecture. With Apple Silicon Macs this test is now broken, since
an M1/M2 Mac would be detected as an iOS target.

For now, we skip this check by looking for a non-existent arm-iphone
architecture. This should be fine since currently we don't support iOS
anyway, and AFAIK Purr Data has never been tested there. If we want to
revive iOS cross compilation at some point, we'll have to devise a
check which will do the right thing on both iOS and macOS arm devices.

Python 2 has been EOL since 2019, and isn't available in the Arch
repositories any more, so continuing to support it has become a major
hassle. The quick and dirty fix is to just build the DISIS cwiid library
without it, which seems to be the only part of the source which still
depends on Python 2 at build time.

This requires various changes throughout the build system. Moreover,
the Mac build now works with both Homebrew (in /usr/local/opt) and
MacPorts (/opt/local), but still prefers Homebrew if it is present.

We also adjusted the embed-MacOSX-dependencies.sh script to look for
libquicktime dependents in the right place ($PD_APP_CONTENTS/Plugins),
so that Gem's QT4 plugins will work using the Homebrew libquicktime
package. Alas, MacPorts doesn't have a working libquicktime package, so
the QT4 plugins are disabled there.

Note that setting the fluidsynth log callbacks doesn't help with these
messages, thus we have to resort to the rather drastic method of temporarily
redirecting stderr to /dev/null.

fluid~: Get rid of the custom settings except gain, better to use the fluidsynth defaults in most cases.

As discussed on the mailing list, it's useful to allow symbol
inputs (other than bang) on pddplink's inlet, to make it easier to
invoke pddplink on a variable symbol (filename, patch, URL) value.

Note that for backward compatibility, a dummy filename argument is still
needed. Please check the updated help patch for details and an example.

This is needed to make builds on Arch work again.

Disable zexy/lpt on Windows for now, it doesn't work there at present and causes a segfault in the regression tests.