Fix Solaris builds to link libnotify with the mapfile

Two pieces:

1. Mapfile flags were being included in the dependency object, used to build tests and tools, instead of just the libnotify_lib object, which on Solaris caused errors about undefined symbols at link-time of the tests and tools.
2. Solaris expects mapfiles in its native syntax, but in Solaris 11.4 will accept the GNU syntax used here, if -zgnu-version-script-compat is also passed to the linker at the same time. (For the meson cc.get_supported_link_arguments test to pass, it also needs to be included when testing --version-script, hence the string concatenation instead of using a list of arguments to be independently tested.)