better build script

build.sh

@@ -18,7 +18,18 @@
 # You should have received a copy of the GNU Affero General Public License
 # along with this program. If not, see https://www.gnu.org/licenses/.
 
+if ! test -n "$1"; then
+	printf "Usage: %s [target]\n" "$0" 1>&2
+	printf "For a list of targets, type 'rustc --print target-list'\n" 1>&2
+	exit 64 # sysexits(3) EX_USAGE
+fi
+
+if ! command -v cargo; then
+  printf "%s: Missing depenency: cargo(1)\n" "$0" 1>&2
+	exit 69 # syexits(3) EX_UNAVAILABLE
+fi
+
 cargo build --release \
 	-Z build-std=std,panic_abort \
 	-Z  build-std-features=panic_immediate_abort \
-	--target x86_64-unknown-linux-gnu
+	--target "$1"