build fails if function-block is disabled

[Jun. T <tak...@kb...>]

If configured without --enable-function-block, build fails as

internal.c:98:35: error: expression is not assignable
   98 |     evaluate_inside_functionblock = FALSE;
      |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^


Either add '#ifdef USE_FUNCTIONBLOCKS' to this line, or modify
datablock.h so that evaluate_inside_functionblock is always a variable.