Lagom+CMake: Propagate dependencies for generated custom targets

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.

Userland/Libraries/LibUnicode/CMakeLists.txt

@@ -7,6 +7,7 @@ set(SOURCES
     Normalize.cpp
     ${UNICODE_DATA_SOURCES}
 )
+set(GENERATED_SOURCES ${CURRENT_LIB_GENERATED})
 
 serenity_lib(LibUnicode unicode)
 target_link_libraries(LibUnicode LibCore)