linux: call g_thread_init() at relevant startup points

According to glib docs, we need to do this if it's at all possible
for us to hit glib on multiple threads.  This may be happening
when we grab plugin metadata from the file thread.

Rather than explicitly depending on gthread all over the place,
just put it in with the GTK dep (since anywhere we're using GTK
we ought to init gthread).

(Note that this is *not* initializing the GDK locking system.)

BUG=18957

Review URL: http://codereview.chromium.org/174264

git-svn-id: http://src.chromium.org/svn/trunk/src/build@24148 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

linux/system.gyp

@@ -9,15 +9,15 @@
       'type': 'settings',
       'direct_dependent_settings': {
         'cflags': [
-          '<!@(pkg-config --cflags gtk+-2.0)',
+          '<!@(pkg-config --cflags gtk+-2.0 gthread-2.0)',
         ],
       },
       'link_settings': {
         'ldflags': [
-          '<!@(pkg-config --libs-only-L --libs-only-other gtk+-2.0)',
+          '<!@(pkg-config --libs-only-L --libs-only-other gtk+-2.0 gthread-2.0)',
         ],
         'libraries': [
-          '<!@(pkg-config --libs-only-l gtk+-2.0)',
+          '<!@(pkg-config --libs-only-l gtk+-2.0 gthread-2.0)',
         ],
       },
     },
@@ -106,23 +106,6 @@
         ],
       },
     },
-    {
-      'target_name': 'gthread',
-      'type': 'settings',
-      'direct_dependent_settings': {
-        'cflags': [
-          '<!@(pkg-config --cflags gthread-2.0)',
-        ],
-      },
-      'link_settings': {
-        'ldflags': [
-          '<!@(pkg-config --libs-only-L --libs-only-other gthread-2.0)',
-        ],
-        'libraries': [
-          '<!@(pkg-config --libs-only-l gthread-2.0)',
-        ],
-      },
-    },
     {
       'target_name': 'x11',
       'type': 'settings',