[NaCl SDK] Check for valid CONFIG parameter to Makefile.

native_client_sdk/src/tools/common.mk

Index: native_client_sdk/src/tools/common.mk
diff --git a/native_client_sdk/src/tools/common.mk b/native_client_sdk/src/tools/common.mk
index 78fdf59c8897132cc1e24f679494bfa30d0dcd84..ac9179010e1b0f230ebeb9111dd4c8982ee96814 100644
--- a/native_client_sdk/src/tools/common.mk
+++ b/native_client_sdk/src/tools/common.mk
@@ -112,6 +112,16 @@ endif
 
 
 #
+# Verify we selected a valid configuration for this example.
+#
+VALID_CONFIGS ?= Debug Release
+ifeq (,$(findstring $(CONFIG),$(VALID_CONFIGS)))
+  $(warning Availbile choices are: $(VALID_CONFIGS))
+  $(error Can not use CONFIG=$(CONFIG) on this example.)
+endif
+
+
+#
 # Note for Windows:
 #   The GCC and LLVM toolchains (include the version of Make.exe that comes
 # with the SDK) expect and are capable of dealing with the '/' seperator.