lib: Build libgnomesoftware as a shared library as well as static

This means it can be linked to the plugins dynamically. This reduces the installed plugin size by 50% (and correspondingly the disk reads and memory for loading them). It doesn’t measurably affect startup time, and overall reduces memory consumption by about 5% (although almost all of that will be read-only mappings of the code).

Signed-off-by: Philip Withnall pwithnall@endlessos.org