HPCC-8819 Allow LOCAL to prefix a definition (like SHARED/EXPORT)

There are some situtations e.g., when you are defining a symbol in a
function/function macro, where it is possible for a definition of a symbol
to fail because it clashes with an existing symbol.  Prefixing the
definition with LOCAL allows you to avoid this ambiguity.

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>