No error when segmap is not found

[ryandesign]

If I specify a custom segment map, but the file is not found, no error is issued. Instead, the app is built with just two segments, Runtime and Main, rather than the normal default set of segments. Perhaps an error should be issued when the specified segment map can't be found so the user knows they made a mistake.

One possible source of confusion is that the given path apparently must be absolute. For example, this does not work:

set_target_properties(app PROPERTIES
    LINK_FLAGS "-Wl,--mac-segments,my.segmap"
)

whereas this does work:

set_target_properties(app PROPERTIES
    LINK_FLAGS "-Wl,--mac-segments,${CMAKE_CURRENT_SOURCE_DIR}/my.segmap"
)

[ryandesign]

the given path apparently must be absolute

at least when using an out-of-source build, which I was.