Fixed VS variable shadowing warning (and added test)

contrib/single_file_libs/build_decoder_test.sh

@@ -12,7 +12,7 @@ IN_FILES="examples/emscripten.c"
 # Emscripten build using emcc.
 emscripten_emcc_build() {
   # Compile the the same example as above
-  CC_FLAGS="-Wall -Wextra -Werror -Os -g0 -flto"
+  CC_FLAGS="-Wall -Wextra -Wshadow -Werror -Os -g0 -flto"
   emcc $CC_FLAGS -s WASM=1 -I. -o $OUT_WASM $IN_FILES
   # Did compilation work?
   if [ $? -ne 0 ]; then
@@ -66,7 +66,7 @@ fi
 echo "Single file decoder creation script: PASSED"
 
 # Compile the generated output
-cc -Wall -Wextra -Werror -Os -g0 -o $OUT_FILE examples/simple.c
+cc -Wall -Wextra -Wshadow -Werror -Os -g0 -o $OUT_FILE examples/simple.c
 # Did compilation work?
 if [ $? -ne 0 ]; then
   echo "Compiling simple.c: FAILED"