can't compile with strict C99 flags

[chriscamacho]

There seems to be issues with anonymous unions and also with casting function pointers to void*

here are the flags I use

CFLAGS:= -Wfatal-errors -pedantic -Wall -Wextra -Werror -std=c99

While this might seem over fussy, its saved me countless hours of debugging frustration!

[RokerHRO]

It seems to be the same problem as in another expression parser: https://github.com/codeplea/tinyexpr/pull/47