build: Build backend libraries before the backend modules themselves
dummy: Build backend library before the module itself
build: Use AS_HELP_STRING instead of the deprecated AC_HELP_STRING
build: Update configure.ac as suggested by autoupdate

Thanks, autoupdate.
build: Remove unused variables from configure.ac
build: Allow backend libraries to have separate LT versions from libfolks.so

With the addition of the dummy backend, backend libraries (such as
libfolks-dummy.so) no longer necessarily have the same stability guarantees
as the core of libfolks. Consequently, they must have separate LT versions.

These versions will be incremented in-line with the core LT version, but
may also be incremented separately. They don’t affect whether libfolks will
load a given backend module (that’s predicated on the core LT current
version); merely whether client code needs to be re-linked against a given
backend library due to an ABI-incompatible change in its backend-specific
API.
dummy: Use backend-specific LT versioning
dummy: Use a map iterator instead of foreach
dummy: Don’t unnecessarily include the local IID in local-ids

local-ids is used for linking to other personas, so having our IID
in there just creates a pointless reflexive link.
dummy: Fix indentation

Bad Philip. Bad naughty evil wicked Philip.