Build fails on RHEL10 / GCC 14 due to ff2_lex implicit declaration

[SiskaPavel]

Hi,

I tried to build the package on RHEL 10 / Oracle Linux 10 with GCC 14, but the build fails with the following error:

ffilter/ffilter_gram.c: In function 'ff2_parse':
ffilter/ffilter_gram.c:69:25: error: implicit declaration of function 'ff2_lex' [-Wimplicit-function-declaration]
   69 | #define yylex           ff2_lex
      |                         ^~~~~~~

It seems the issue is caused by stricter rules in newer GCC versions.