Make this work with either brew or macports and emit diagnostic when (#419)

neither is found rather than just blowing up.

build_release_macos.sh

@@ -32,9 +32,18 @@ fi
 echo "Arch: $ARCH"
 echo "BUILD_TARGET: $BUILD_TARGET"
 
-brew --prefix libiconv
-brew --prefix zstd
-export LIBRARY_PATH=$LIBRARY_PATH:$(brew --prefix zstd)/lib/
+if which -s brew; then
+	brew --prefix libiconv
+	brew --prefix zstd
+	export LIBRARY_PATH=$LIBRARY_PATH:$(brew --prefix zstd)/lib/
+elif which -s port; then
+	port install libiconv
+	port install zstd
+	export LIBRARY_PATH=$LIBRARY_PATH:/opt/local/lib
+else
+	echo "Need either brew or macports to successfully build deps"
+	exit 1
+fi
 
 WD="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 cd $WD/deps