Change location of private key file used to sign distribution packages

- If the environment variable OSXFUSE_PRIVATE_KEY is defined, the file it
  refers to is used to sign distribution packages
- If OSXFUSE_PRIVATE_KEY is not defined, the script uses the file
  $HOME/.osxfuse_private_key to sign distribution packages
- If the private key file could not be located, an error message is displayed

build.sh

@@ -15,7 +15,12 @@ export PATH
 #         Therefore, do not set M_CONF_TMPDIR to such a directory.
 #
 readonly M_CONF_TMPDIR=/tmp
-readonly M_CONF_PRIVKEY=/etc/macfuse/private.der
+
+if [ -n "$OSXFUSE_PRIVATE_KEY" ]; then
+    readonly M_CONF_PRIVKEY="$OSXFUSE_PRIVATE_KEY"
+else
+    readonly M_CONF_PRIVKEY="$HOME/.osxfuse_private_key"
+fi
 
 # Other constants
 #