Prepare to use generated shim headers for speex

speex.gyp

Index: speex.gyp
===================================================================
--- speex.gyp	(revision 160092)
+++ speex.gyp	(working copy)
@@ -95,13 +95,30 @@
         {
           'target_name': 'libspeex',
           'type': 'none',
+          'variables': {
+            'headers_root_path': 'include',
+            'header_filenames': [
+              'speex/speex_types.h',
+              'speex/speex_callbacks.h',
+              'speex/speex_config_types.h',
+              'speex/speex_stereo.h',
+              'speex/speex_echo.h',
+              'speex/speex_preprocess.h',
+              'speex/speex_jitter.h',
+              'speex/speex.h',
+              'speex/speex_resampler.h',
+              'speex/speex_buffer.h',
+              'speex/speex_header.h',
+              'speex/speex_bits.h',
+            ],
+          },
+          'includes': [
+            '../../build/shim_headers.gypi',
+          ],
           'direct_dependent_settings': {
             'cflags': [
               '<!@(pkg-config --cflags speex)',
             ],
-            'defines': [
-              'USE_SYSTEM_SPEEX',
-            ],
           },
           'link_settings': {
             'ldflags': [