rustup-init.sh: Fall back to less secure download

If we're using OS X less than 10.13 or any platform where
curl or wget fail to advertise the required command line arguments
then fall back to not passing the flags to force TLSv1.2

This should fix rust-lang#1794

Signed-off-by: Daniel Silverstone <dsilvers@digital-scurf.org>

rustup-init.sh

@@ -371,12 +371,49 @@ downloader() {
     if [ "$1" = --check ]; then
         need_cmd "$_dld"
     elif [ "$_dld" = curl ]; then
-        curl --proto '=https' --tlsv1.2 --silent --show-error --fail --location "$1" --output "$2"
+        if ! check_help_for curl --proto --tlsv1.2; then
+            echo "Warning: Not forcing TLS v1.2, this is potentially less secure"
+            curl --silent --show-error --fail --location "$1" --output "$2"
+        else
+            curl --proto '=https' --tlsv1.2 --silent --show-error --fail --location "$1" --output "$2"
+        fi
     elif [ "$_dld" = wget ]; then
-        wget --https-only --secure-protocol=TLSv1_2 "$1" -O "$2"
+        if ! check_help_for wget --https-only --secure-protocol; then
+            echo "Warning: Not forcing TLS v1.2, this is potentially less secure"
+            wget "$1" -O "$2"
+        else
+            wget --https-only --secure-protocol=TLSv1_2 "$1" -O "$2"
+        fi
     else
         err "Unknown downloader"   # should not reach here
     fi
 }
 
+check_help_for() {
+    local _cmd
+    local _arg
+    local _ok
+    _cmd="$1"
+    _ok="y"
+    shift
+
+    # If we're running on OS-X, older than 10.13, then we always
+    # fail to find these options to force fallback
+    if check_cmd sw_vers; then
+        if [ "$(sw_vers -productVersion | cut -d. -f2)" -lt 13 ]; then
+            # Older than 10.13
+            echo "Warning: Detected OS X platform older than 10.13"
+            _ok="n"
+        fi
+    fi
+
+    for _arg in "$@"; do
+        if ! "$_cmd" --help | grep -q -- "$_arg"; then
+            _ok="n"
+        fi
+    done
+
+    test "$_ok" = "y"
+}
+
 main "$@" || exit 1