lib: NOTMUCH_DEPRECATED macro also for older compilers

lib: NOTMUCH_DEPRECATED macro also for older compilers

Some compilers (older than gcc 4.5 and clang 2.9) do support
__attribute__ ((deprecated)) but not
__attribute__ ((deprecated("message"))).

Check if clang version is at least 3.0, or gcc version
is at least 4.5 to define NOTMUCH_DEPRECATED as the
latter variant above. Otherwise define NOTMUCH_DEPRECATED
as the former variant above.

For a bit simpler implementation clang 2.9 is not included
to use the newer variant. It is just one release, and the
older one works fine. Clang 3.0 was released around 2011-11
and gcc 5.1 2015-04-22 (therefore newer macro for gcc 4.5+)