Avoid unknown warning warnings and fix -Werror on macosx.

clang on macosx does not support -Wunused-but-set-variable; adding the flag
causes additional warnings about the flag. As a more generalized fix, use
-Werror when checking compiler flag support in order to avoid using
unsupported warning flags.

Change-Id: I2529862e211f880d56491eac3b9fa90fff1aa5c3

build/make/configure.sh

@@ -327,7 +327,7 @@ EOF
 
 check_cflags() {
     log check_cflags "$@"
-    check_cc "$@" <<EOF
+    check_cc -Werror "$@" <<EOF
 int x;
 EOF
 }
@@ -341,7 +341,7 @@ check_cxxflags() {
 int x;
 EOF
       ;;
-      *) check_cxx "$@" <<EOF
+      *) check_cxx -Werror "$@" <<EOF
 int x;
 EOF
       ;;