Clean up compiler warnings.

* src/builtin.c (m4_patsubst): Drop dead store.
* src/eval.c (unary_term): Likewise.
* src/input.c (match_input): Likewise.
* src/symtab.c (symtab_debug): Drop useless casts to void.
* src/m4.c (main): Add hints to aid clang analysis.

Signed-off-by: Eric Blake <eblake@redhat.com>
(cherry picked from commit dadaf5f2d58ffd484bd5141fc808552b8f17467d)
6 files changed