Build: Offsets lib as OBJECT not STATIC

Use cmake to get generated path of lib instead of predefined absolute
path.  This is needed to eventually be able to have an
architecture-out-of-tree build.  The path "OFFSETS_O_PATH" is not
generated for sources out-of-tree and therefore must be replaced by the
cmake generator $<TARGET_OBJECTS:offsets>.

The following lines are needed while we are using a Cmake version <
3.12.3:

target_include_directories(.....
target_compile_options(...
target_compile_definitions(....

This is due to the cmake feature "Linking Object Libraries" not
supported until 3.12:
https://cmake.org/cmake/help/v3.12/command/target_link_libraries.html#linking-object-libraries



Signed-off-by: Klaus Petersen <kape@oticon.com>