Make gtk2/gtk3 compile time switchable.

build/linux/system.gyp

Index: build/linux/system.gyp
diff --git a/build/linux/system.gyp b/build/linux/system.gyp
index df651fbd7ec16c392d1062f2f9dfebdbf408d29e..9d9ab32e3800551f868ca51de0a0f2f89f6c32c7 100644
--- a/build/linux/system.gyp
+++ b/build/linux/system.gyp
@@ -128,7 +128,7 @@
           ],
         },
         {
-          'target_name': 'gtk',
+          'target_name': 'gtk2',
           'type': 'none',
           'toolsets': ['host', 'target'],
           'variables': {
@@ -169,7 +169,7 @@
           ],
         },
         {
-          'target_name': 'gtkprint',
+          'target_name': 'gtkprint2',
           'type': 'none',
           'conditions': [
             ['_toolset=="target"', {
@@ -423,6 +423,74 @@
         }
       ],  # targets
     }],
+    ['use_gtk3==1', {
+      # Hide GTK3 and related dependencies when use_gtk3==0 because the user
+      # might not have the GTK3 headers yet.
+      'targets': [
+        {
+          'target_name': 'gtk3',
+          'type': 'none',
+          'toolsets': ['host', 'target'],
+          'variables': {
+            # gtk requires gmodule, but it does not list it as a dependency
+            # in some misconfigured systems.
+            'gtk_packages': 'gmodule-2.0 gtk+-3.0 gthread-2.0',
+          },
+          'conditions': [
+            ['_toolset=="target"', {
+              'all_dependent_settings': {
+                'cflags': [
+                  '<!@(<(pkg-config) --cflags <(gtk_packages))',
+                ],
+              },
+              'link_settings': {
+                'ldflags': [
+                  '<!@(<(pkg-config) --libs-only-L --libs-only-other <(gtk_packages))',
+                ],
+                'libraries': [
+                  '<!@(<(pkg-config) --libs-only-l <(gtk_packages))',
+                ],
+              },
+            }, {
+              'all_dependent_settings': {
+                'cflags': [
+                  '<!@(pkg-config --cflags <(gtk_packages))',
+                ],
+              },
+              'link_settings': {
+                'ldflags': [
+                  '<!@(pkg-config --libs-only-L --libs-only-other <(gtk_packages))',
+                ],
+                'libraries': [
+                  '<!@(pkg-config --libs-only-l <(gtk_packages))',
+                ],
+              },
+            }],
+          ],
+        },
+        {
+          'target_name': 'gtkprint3',
+          'type': 'none',
+          'conditions': [
+            ['_toolset=="target"', {
+              'direct_dependent_settings': {
+                'cflags': [
+                  '<!@(<(pkg-config) --cflags gtk+-unix-print-3.0)',
+                ],
+              },
+              'link_settings': {
+                'ldflags': [
+                  '<!@(<(pkg-config) --libs-only-L --libs-only-other gtk+-unix-print-3.0)',
+                ],
+                'libraries': [
+                  '<!@(<(pkg-config) --libs-only-l gtk+-unix-print-3.0)',
+                ],
+              },
+            }],
+          ],
+        },
+      ],
+    }],
     ['use_x11==1 and chromeos==0', {
       'targets': [
         {