- As a sideeffect of the patch to generate lex_hash.h only once
on the machine where the source dist was produced, a problem
was found when compiling a mysqld without partition support - it 
would crash when looking up the lex symbols due to mismatch between
lex.h and the generated lex_hash.h

 - Remove the ifdef for partition in lex.h 
 - Fix minor problem with"EXPLAIN PARTITION" when not compiled with
   partition(existed also without the above patch)
 - Add test case that will be run when we don't have partition
   support compiled into mysqld
 - Return error ER_FEATURE_DISABLED if user tries to use PARTITION
   when there is no support for it.