Two fixes

[Tomash Brechko]

Hello,

Here come two fixes.  First makes "flush_all 0" work (protocol
documentation says "flush_all" is a short-cut for it, but explicit
zero never worked).  Second fixes --enable-threads for configure:
before both --enable-threads and --disable-threads did the same, and
on some platforms memcached was linked with libpthread even when
threads weren't enabled, which could substitute a number of libc
functions with their thread-aware versions.


-- 
   Tomash Brechko