GDB: Only build for "unix:" connections if AF_LOCAL is supported.

GDB: Only build for "unix:" connections if AF_LOCAL is supported.

Commit f19c7ff839d7a32ebb48482ae7d318fb46ca823d added a new member to the
prefixes array which included a use of the symbol AF_LOCAL.   Unfortunately,
not all systems declare this symbol.    This change only compiles the "unix:"
member if the system knows about AF_LOCAL.

gdb/ChangeLog:

* configure.ac: New test HAVE_AF_LOCAL
* common/netstuff.c (parse_connection_spec) [prefixes]: Only compile "unix:"
   if HAVE_AF_LOCAL is true.
* configure: regenerate.
* config.in: regenerate.