GRUB 2.00 build and flex-2.5.37

[Bruce Dubbs]

The latest flex makes the GRUB-2.00 build fail.  I can work around the 
issues with --disable-werror, but grub-core/script/yylex.l causes two 
warnings that do not work with -Werror.

./grub-core/script/yylex.l: At top level:
grub_script.yy.c:2351:13: error: 'yy_fatal_error' defined but not used

This can be fixed by removing  #define YY_FATAL_ERROR(msg) in yylex.l.

grub_script.yy.c: In function 'yy_scan_bytes':
grub_script.yy.c:2296:17: error: comparison between signed and unsigned 
integer expressions [-Werror=sign-compare]

This one is a little more difficult because it is generated by flex.  I 
don't know how to fix this except with a sed after grub_script.yy.c is 
generated by flex.

The gcc version -s 4.7.1.

  -- Bruce