chore(*) install ngx_wasm_rs along with nginx

There's a tricky thing going on. Since Wasmer itself is written in Rust and we
build using a precompiled libwasmer.a generated with a different Rust version,
if we link both libwasmer.a and libngx_wasm_rs.a, we get a link failure with
some duplicated incompatible symbols [1][2].

For this reason, when we enable ngx_wasm_rs with Wasmer, we build
it as a .so file rather than a statically-linked .a file.

With this commit, we can install cdylibs under nginx's `--prefix` at `/lib`
(e.g. `/usr/local/nginx/lib`, next to `/usr/local/nginx/sbin/nginx`) and
ensure that the `nginx` binary can find it.

See here for more info:

[1] https://users.rust-lang.org/t/linking-more-than-one-rustc-compiled-static-libraries-in-a-c-project/89778/2
[2] #259 (comment)

auto/cargo

@@ -25,6 +25,7 @@ if [ $ngx_wasm_rc = 0 ]; then
     if [ "$ngx_wasm_cargo_crate_type" = "cdylib" ]; then
         ngx_feature_libs="\
             -Wl,-rpath $ngx_wasm_cargo_target_dir \
+            -Wl,-rpath $NGX_PREFIX/lib \
             -L$ngx_wasm_cargo_target_dir \
             -l$ngx_wasm_cargo_lib_name"
     else
@@ -41,6 +42,28 @@ if [ $ngx_wasm_rc = 0 ]; then
             have=$d value=1 . auto/define
         done
 
+        if [ "$ngx_wasm_cargo_crate_type" = "cdylib" ]; then
+            ngx_wasm_cargo_cdylib="$ngx_wasm_cargo_target_dir/lib$ngx_wasm_cargo_lib_name.so"
+            ngx_wasm_cargo_cdylibs="$ngx_wasm_cargo_cdylibs $ngx_wasm_cargo_cdylib"
+
+            if ! grep -q ngx_wasm_cargo_cdylib auto/install; then
+                cat << EOF >> auto/install
+
+for ngx_wasm_cargo_cdylib in \$ngx_wasm_cargo_cdylibs
+do
+
+cat << END                                                    >> \$NGX_MAKEFILE
+
+	test -d '\\\$(DESTDIR)\$NGX_PREFIX/lib' \\
+		|| mkdir -p '\\\$(DESTDIR)\$NGX_PREFIX/lib'
+	cp \$ngx_wasm_cargo_cdylib '\\\$(DESTDIR)\$NGX_PREFIX/lib'
+END
+
+done
+EOF
+            fi
+        fi
+
     else
         echo "$0: error: failed testing library $ngx_wasm_cargo_lib_name."
     fi