configure.ac: Restructure uuid library detection.

configure.ac: Restructure uuid library detection.

The 'uuid' library functionality may be located in a separate 'uuid'
library, but it may also be included in the standard C library. This is
the case on Mac OS X.

Instead of relying on the uuid code being located in a 'uuid' library,
we restructure the code so that it checks different libraries, in order
for the 'uuid_generate' function. This code is easy to extend if we
would find a new configuration where uuid_generate is located in a
different library.