MacOS 64bit support

[Yassine Chbani]

Hi all,

Lilypond does not currently offer 64bit binaries for MacOS on its download
page <http://lilypond.org/doc/v2.19/Documentation/web/macos-x>. These 64bit
binaries are however necessary to run Lilypond on the latest macOS Catalina
release (that removed 32bit support).

I am currently setting up GUB to try and compile Lilypond on the darwin-x86_64
platform, is there any particular issue I should be aware of regarding
64bit compiling for MacOS targets?

Thanks,
Yassin