We have logic for serenity_generated_sources which works well for source
files that are specified in GENERATED_SOURCES prior to calling
serenity_lib or serenity_bin. However, code generated with
invoke_generator, and the LibWeb generators do not always follow the
pattern of the IDL and GML files.
For the LibWeb generators, we can just add_dependencies to LibWeb at the
time we declare the generate_Foo custom target. However for LibLocale,
LibTimeZone, and LibUnicode, we don't have the name of the target
available, so export the name in a variable to set into
GENERATED_SOURCES.
To make this work for Lagom, we need to make sure that lagom_lib and
serenity_bin in Lagom/CMakeLists.txt call serenity_generated_sources on
the target.
This enables the Xcode generator on macOS hosts, at least for Lagom.
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