build: fix -Wstrict-prototypes warnings

We disabled -Wstrict-prototypes in commit
db9b1df0e47996ff8aaea468a11e1e97f64ee126 .
Fix compiler warnings.

Signed-off-by: Thomas Haller <thaller@redhat.com>
build: refactor compiler_warnings.m4 macro

NM_COMPILER_WARNINGS still works the same, but
rename variables to have a CFLAGS_* prefix.

Also cleanup the construction of CFLAGS by appending
to CFLAGS_MORE_WARNINGS variable instead of appending
to CFLAGS, and resetting to SAVE_CFLAGS.

Signed-off-by: Thomas Haller <thaller@redhat.com>