Previously didn't add the source to the target if a source of the same
name already existed. This was janky because these files would be
considered the same: src/foo.cpp src/armv4t/bar/baz/foo.cpp, even though
they really shouldn't. What should happen instead, is that the symbols
of the architecture-specific code should not be overridden by the common
implementation regardless of where the file is placed. This means that
if the files src/foo.cpp and src/armv4t/bar.cpp contain implementations
of the function foo, the armv4t implementation will be exported
even though it uses a different filename from the common
implementation. This commit implements this behaviour by using
the way symbols are naturally resolved. Multiple smaller
libraries are built for each architecture dependent code.
Afterwards the libraries are linked into one, with the arch specific
libraries linked first.
CONFIGURE_DEPENDS option was added in CMake v3.12. This option allows the
build system to automatically re-run CMake if the glob changes, solving the
major issue with globbing. May have a performance impact, but it should be
negligible compared to the time spent building.
