Use a more general method of aliasing md5sum

Rather than checking the platform or operating system, simply check if
the command exists. If the check fails, check if the md5 command exists.
If that fails as well, cancel the build script. Otherwise, assign the
alias as before.

scripts/remap.sh

@@ -24,11 +24,16 @@ if [ ! -f  "$jarpath.jar" ]; then
 fi
 
 # OS X & FreeBSD don't have md5sum, just md5 -r
-if [[ "$OSTYPE" == "darwin"* || "$(uname)" == "FreeBSD" ]]; then
+command -v md5sum >/dev/null 2>&1 || {
+    command -v md5 >/dev/null 2>&1 && {
         shopt -s expand_aliases
         alias md5sum='md5 -r'
-   echo "Using an alias for md5sum on macOS and/or FreeBSD"
-fi
+        echo "md5sum command not found, using an alias instead"
+    } || {
+        echo >&2 "No md5sum or md5 command found"
+        exit 1
+    }
+}
 
 checksum=$(md5sum "$jarpath.jar" | cut -d ' ' -f 1)
 if [ "$checksum" != "$minecrafthash" ]; then