I also faced this issue and this is how I fixed it:
ada-exp.y includes file ada-lex.c which contains the definition for function lexer_init.
But if you don’t have flex installed initially, then it may generate an empty ada-lex.c (from ada-lex.l). And even if you do make clean, it will not clean the generated ada-lex.c, i.e., it will remain empty upon next compile command as well.
So to fix this error, you can delete all such intermediate .c files (in your case, building with fresh .tar.gz) and do the make again.
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