…`libcommonstr.la` recipe

A smaller helper library than `libcommonclient.la` which is sometimes an
overkill, e.g. in `nut-scanner` where the `libupsclient.so/.dll` is loaded
dynamically for "Old NUT" scans and there is no specific use for object
files included into `libcommonclient.la`.

More use-cases with DMF codebase (currently not yet upstreamed).

Combined from a series of commits like
* 3ba6b57
* c418728
* 5f16b7b
* bc4cd95

Signed-off-by: Jim Klimov <[email protected]>

…rkupstools#2463, networkupstools#2478]

Signed-off-by: Jim Klimov <[email protected]>