cmake: Fix source-specific compile definitions

set_source_files_properties() overwrites previously specified
properties and does not have a way to append, which is bad when
e.g. conditionally adding multiple compile definitions to a file.
In this particular case, for main.cpp -DHAVE_UMASK would overwrite
-DEMBED_DATA, and thus resources would not be loaded.

Use set_property(SOURCE... APPEND PROPERTY...) instead.