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.

m4/compiler_warnings.m4

@@ -2,13 +2,13 @@ AC_DEFUN([NM_COMPILER_WARNINGS],
 [AC_ARG_ENABLE(more-warnings,
 	AS_HELP_STRING([--enable-more-warnings], [Maximum compiler warnings]),
 	set_more_warnings="$enableval",set_more_warnings=yes)
-AC_MSG_CHECKING(for more warnings, including -Werror)
+AC_MSG_CHECKING(for more warnings)
 if test "$GCC" = "yes" -a "$set_more_warnings" != "no"; then
 	AC_MSG_RESULT(yes)
-	CFLAGS="-Wall -Werror -std=gnu89 $CFLAGS"
+	CFLAGS="-Wall -std=gnu89 $CFLAGS"
 
 	for option in -Wshadow -Wmissing-declarations -Wmissing-prototypes \
-		      -Wdeclaration-after-statement -Wstrict-prototypes \
+		      -Wdeclaration-after-statement \
 		      -Wfloat-equal -Wno-unused-parameter -Wno-sign-compare \
 		      -fno-strict-aliasing -Wno-unused-but-set-variable \
 		      -Wundef; do
@@ -26,6 +26,7 @@ if test "$GCC" = "yes" -a "$set_more_warnings" != "no"; then
 		unset SAVE_CFLAGS
 	done
 	unset option
+	CFLAGS="$CFLAGS -Werror"
 else
 	AC_MSG_RESULT(no)
 fi