autostart: revision for snap packaging

Adjust JAMI_DATA_PATH (and in turn JAMI_DATA_DIR) for snap after any
cmake 'install' command definitions that refer to it, so that the
version compiled into the package using target_compile_definitions
will have the right value for our snap package.  With this particular
configuration, we don't have to check for JAMI_DATA_DIR during runtime
since it will have the right value set at build time, and the path it
points to (/snap/jami/current/...) is actually stable across updates,
so we don't have to worry about correcting the autostart desktop file
symlink after each upgrade.

Note: as the comments in CMakeLists.txt mention, it is crucial that
JAMI_DATA_PATH is only adjusted after all 'install' commands that
refer to it, because its snap-specific value isn't meant to be used
during build time as an install destination.  Also, that the call to
target_compile_definitions must come after the JAMI_DATA_PATH change
described earlier.

Co-authored-by: ababi <albert.babi@savoirfairelinux.com>

Gitlab: #262
Change-Id: I07896be8195c336833bcd4a84b918276eddbe159