libuuid: support non-cached scenarios (when -lpthread is unavailable)

libuuid: support non-cached scenarios (when -lpthread is unavailable)

This patch makes the dependence on pthread optional for libuuid. In
certain cases, such as Buildroot Linux, uClibc-ng, and very low
resource systems, libpthread may be unavailable.

If libuuid is compiled without pthread, it will not use a local cache
and will instead request a UUID from uuidd for each call. This may
result in less efficient performance, but the UUIDs generated will
still be unique and reliable.

On minimalistic systems, it is highly likely that uuidd will not be
installed, making this change important for portability and robust
code.

Addresses: https://github.com/util-linux/util-linux/pull/3375
Signed-off-by: Karel Zak <kzak@redhat.com>