iosbuild: Add --test-link argument.

Shortcut arg for --extra-configure-args --enable-examples. Enables
the examples, and thus ensures that all versions of libvpx that
iosbuild.sh produces can actually be linked.

Change-Id: I2ddda094361bf0ac77f8d2ae542e4dc7b2cab158

build/make/iosbuild.sh

@@ -199,6 +199,8 @@ cat << EOF
     --show-build-output: Show output from each library build.
     --targets <targets>: Override default target list. Defaults:
          ${TARGETS}
+    --test-link: Confirms all targets can be linked. Functionally identical to
+                 passing --enable-examples via --extra-configure-args.
     --verbose: Output information about the environment and each stage of the
                build.
 EOF
@@ -237,6 +239,9 @@ while [ -n "$1" ]; do
     --show-build-output)
       devnull=
       ;;
+    --test-link)
+      EXTRA_CONFIGURE_ARGS="${EXTRA_CONFIGURE_ARGS} --enable-examples"
+      ;;
     --targets)
       TARGETS="$2"
       shift