build: don't use -Wstrict-prototypes (bgo #584947)

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.