examples/ubuntu: Use pkg-config to pull correct package dependencies

When adding dependecies on installed libraries, use their .pc files
to specify additional library includes, defines and lib dependencies
by listing packagaes in 'deps' variable in weave.gyp and then simple
rules to pull out dependencies from the package config files.

Change-Id: I09ab1329b2fc8cf4a6a94b7e96dacf1e51cbfbdd
Reviewed-on: https://weave-review.googlesource.com/1256
Reviewed-by: Johan Euphrosine <proppy@google.com>
Reviewed-by: Vitaly Buka <vitalybuka@google.com>

libweave/examples/ubuntu/weave.gyp

diff --git a/libweave/examples/ubuntu/weave.gyp b/libweave/examples/ubuntu/weave.gyp
index 81b492b..a51db93 100644
--- a/libweave/examples/ubuntu/weave.gyp
+++ b/libweave/examples/ubuntu/weave.gyp
@@ -3,7 +3,29 @@
     {
       'target_name': 'weave',
       'type': 'executable',
-      'cflags': ['-pthread', '-I/usr/include/libnl3'],
+      'variables': {
+        'deps': [
+          'avahi-client',
+          'expat',
+          'libcurl',
+          'libcrypto',
+          'libnl-3.0',
+          'libnl-route-3.0',
+          'openssl',
+        ]
+      },
+      'cflags': [
+        '>!@(pkg-config >(deps) --cflags)',
+        '-pthread',
+      ],
+      'link_settings': {
+        'ldflags+': [
+          '>!@(pkg-config >(deps) --libs-only-L --libs-only-other)',
+        ],
+        'libraries+': [
+          '>!(pkg-config >(deps) --libs-only-l)',
+        ],
+      },
       'sources': [
         'avahi_client.cc',
         'bluez_client.cc',
@@ -21,16 +43,8 @@
       ],
       'libraries': [
         '-levent',
-        '-lcrypto',
-        '-lexpat',
-        '-lcurl',
-        '-lpthread',
-        '-lssl',
-        '-lavahi-common',
-        '-lavahi-client',
         '-levent_openssl',
-        '-lnl-3',
-        '-lnl-route-3',
+        '-lpthread',
       ]
     }
   ]