build: don't use -Wstrict-prototypes (bgo #584947)
First, -Wstrict-prototypes wasn't actually getting used because -Werror was already in CFLAGS and AC_TRY_COMPILE doesn't produce main() functions with valid strict prototypes. Suck. But even fixing that, the WiMAX SDK won't build with the flag, so just rip it out.
Showing with 4 additions and 3 deletions