[CMake] Shared library from sources in several subdirectories

[Bruce Stephens]

Any suggestions on how to organise that? Presumably there are
lots of examples in (for example) KDE, but I'm not familiar enough
with that codebase to be able to find them easily.

Currently our GNU Make build builds static libraries in the subdirectories,
then those are put into a big static library and that's linked into a shared
library. (The last step uses a shell script which knows how to do that for
various platforms.)

My first attempt uses add_library(... OBJECT ...) in the subdirectories,
then add_library(... SHARED $<TARGET_OBJECTS:...> $<TARGET_OBJECTS:...>)
to combine them.

That works, though it means repeating the same information (more or less)
twice
since for each add_subdirectory() there'll be a $<TARGET_OBJECTS:>. Also
the dependent libraries need to be added in the top-level but they're
triggered
by the subdirectory (by the looks of it I can't use target_link_libraries()
on an OBJECT
library). It works, though, and didn't take me very long to do, and the
result looks
clean and relatively easy to follow.

Any better ways to do this? (In the particular case I'm looking at,
splitting the
libraries and just having one per subdirectory might make sense, but not in
all
cases.)
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