It is not correct to say -lpthread.  The correct option is -pthread,
which may have sundry other effects on code generation etc.  It needs
to be passed both to compilation and linking.

Fix the configure test to test -pthread, and plumb the resulting flag
through to PTHREAD_{CFLAGS,LDFLAGS} in Tools.mk; also substitute
PTHREAD_LIBS (although this will currently always be empty).
Remove PTHREAD_LIBS setting from StdGNU.mk.

Fix the one user (libxc) to use PTHREAD_{CFLAGS,LDFLAGS} too.

There are still some other users in tree which pass -pthread or
-lpthread by adding it as a literal to their own compiler options.
These will be fixed in a later patch.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Roger Pau Monne <roger.pau@entel.upc.edu>
Acked-by: Roger Pau Monne <roger.pau@entel.upc.edu>
Committed-by: Ian Jackson <Ian.Jackson@eu.citrix.com>