Fix mismatch between printf format strings and arguments in some
solosolver verbose diagnostics in X mode. Also added gcc-specific
prototypes with __attribute__((format)) to ensure they all get checked
in future.

Spotted by Arun Giridhar; segfault without this fix is reproducible by
'solosolver -v 3x3x:7_9e4_1c7d3e3d1b2_4e2c6e5_6b1d8e5d9c8_2e9_5'.

[originally from svn r9151]