build: allow use of system-installed brotli

brotli is available as a shared library since 2016, so it makes sense
to allow its use as a system-installed version.

Some of the infrastructure was in place already (node.gyp and
node.gypi), but some bits in the configure script here were missing.

Add them, keeping the default as before, to use the bundled version.

Refs: google/brotli#421
Signed-off-by: André Draszik <git@andred.net>

configure.py

@@ -301,6 +301,27 @@
     dest='shared_zlib_libpath',
     help='a directory to search for the shared zlib DLL')
 
+shared_optgroup.add_option('--shared-brotli',
+    action='store_true',
+    dest='shared_brotli',
+    help='link to a shared brotli DLL instead of static linking')
+
+shared_optgroup.add_option('--shared-brotli-includes',
+    action='store',
+    dest='shared_brotli_includes',
+    help='directory containing brotli header files')
+
+shared_optgroup.add_option('--shared-brotli-libname',
+    action='store',
+    dest='shared_brotli_libname',
+    default='brotlidec,brotlienc',
+    help='alternative lib name to link to [default: %default]')
+
+shared_optgroup.add_option('--shared-brotli-libpath',
+    action='store',
+    dest='shared_brotli_libpath',
+    help='a directory to search for the shared brotli DLL')
+
 shared_optgroup.add_option('--shared-cares',
     action='store_true',
     dest='shared_cares',
@@ -1692,6 +1713,7 @@ def make_bin_override():
 configure_library('zlib', output)
 configure_library('http_parser', output)
 configure_library('libuv', output)
+configure_library('brotli', output, pkgname=['libbrotlidec', 'libbrotlienc'])
 configure_library('cares', output, pkgname='libcares')
 configure_library('nghttp2', output, pkgname='libnghttp2')
 configure_v8(output)