Fix enum Value issue with gcc 4.4

Fix enum Value issue with gcc 4.4

Louis Zulli reports a miscompile with g++-4.4 from MacPorts.

Namely enum Value is compiled as unsigned instead of signed integer
and this yields an issue in score_string() where float(v) is incorrectly
casted when Value v is negative.

This patch ensure that compiler choses a signed variable to store a Value.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>