add need_one_of and need tar for tar.gz extension

www/install.sh

@@ -51,6 +51,19 @@ need() {
   fi
 }
 
+need_one_of() {
+  has_one="false"
+  for cmd in "$@"; do
+    if command -v "$cmd" > /dev/null 2>&1; then
+      has_one="true"
+      break
+    fi
+  done
+  if [ $has_one = "false" ]; then
+    err "need one of $* (none of the commands were found)"
+  fi
+}
+
 download() {
   url="$1"
   output="$2"
@@ -104,13 +117,10 @@ while test $# -gt 0; do
   shift
 done
 
-command -v curl > /dev/null 2>&1 ||
-  command -v wget > /dev/null 2>&1 ||
-  err "need wget or curl (command not found)"
-
+need_one_of curl wget
+need_one_of tar unzip
 need mkdir
 need mktemp
-need tar
 
 if [ -z "${tag-}" ]; then
   need grep
@@ -157,8 +167,8 @@ if [ -z "${target-}" ]; then
 fi
 
 case $target in
-  x86_64-pc-windows-msvc) extension=zip; need unzip;;
-  *)                      extension=tar.gz;;
+  x86_64-pc-windows-msvc) extension=zip;    need unzip;;
+  *)                      extension=tar.gz; need tar;;
 esac
 
 archive="$releases/download/$tag/$crate-$tag-$target.$extension"