Linux: add GYP variable to use the system libpng.

third_party/libpng/libpng.gyp

Index: third_party/libpng/libpng.gyp
diff --git a/third_party/libpng/libpng.gyp b/third_party/libpng/libpng.gyp
index 63bd9d7a7895d9b17e1c65e7a2740996b38822c7..03320e8f92c5e72e91ecbc257107b6c220db8758 100644
--- a/third_party/libpng/libpng.gyp
+++ b/third_party/libpng/libpng.gyp
@@ -6,55 +6,88 @@
   'includes': [
     '../../build/common.gypi',
   ],
-  'targets': [
-    {
-      'target_name': 'libpng',
-      'type': '<(library)',
-      'dependencies': [
-        '../zlib/zlib.gyp:zlib',
+  'variables': {
+    'use_system_libpng%': 0,
+  },
+  'conditions': [
+    ['use_system_libpng==0', {
+      'targets': [
+        {
+          'target_name': 'libpng',
+          'type': '<(library)',
+          'dependencies': [
+            '../zlib/zlib.gyp:zlib',
+          ],
+          'defines': [
+            'CHROME_PNG_WRITE_SUPPORT',
+            'PNG_USER_CONFIG',
+          ],
+          'msvs_guid': 'C564F145-9172-42C3-BFCB-6014CA97DBCD',
+          'sources': [
+            'png.c',
+            'png.h',
+            'pngconf.h',
+            'pngerror.c',
+            'pnggccrd.c',
+            'pngget.c',
+            'pngmem.c',
+            'pngpread.c',
+            'pngread.c',
+            'pngrio.c',
+            'pngrtran.c',
+            'pngrutil.c',
+            'pngset.c',
+            'pngtrans.c',
+            'pngusr.h',
+            'pngvcrd.c',
+            'pngwio.c',
+            'pngwrite.c',
+            'pngwtran.c',
+            'pngwutil.c',
+          ],
+          'direct_dependent_settings': {
+            'include_dirs': [
+              '.',
+            ],
+            'defines': [
+              'CHROME_PNG_WRITE_SUPPORT',
+              'PNG_USER_CONFIG',
+            ],
+          },
+          'export_dependent_settings': [
+            '../zlib/zlib.gyp:zlib',
+          ],
+          'conditions': [
+            ['OS!="win"', {'product_name': 'png'}],
+          ],
+        },
+      ]
+    }, {
+      'targets': [
+        {
+          'target_name': 'libpng',
+          'type': '<(library)',
+          'dependencies': [
+            '../zlib/zlib.gyp:zlib',
+          ],
+          'defines': [
+            'USE_SYSTEM_LIBPNG',
+          ],
+          'direct_dependent_settings': {
+            'cflags': [
+              '<!@(pkg-config --cflags libpng)',
+            ],
+          },
+          'link_settings': {
+            'ldflags': [
+              '<!@(pkg-config --libs-only-L --libs-only-other libpng)',
+            ],
+            'libraries': [
+              '<!@(pkg-config --libs-only-l libpng)',
+            ],
+          },
+        },
       ],
-      'defines': [
-        'CHROME_PNG_WRITE_SUPPORT',
-        'PNG_USER_CONFIG',
-      ],
-      'msvs_guid': 'C564F145-9172-42C3-BFCB-6014CA97DBCD',
-      'sources': [
-        'png.c',
-        'png.h',
-        'pngconf.h',
-        'pngerror.c',
-        'pnggccrd.c',
-        'pngget.c',
-        'pngmem.c',
-        'pngpread.c',
-        'pngread.c',
-        'pngrio.c',
-        'pngrtran.c',
-        'pngrutil.c',
-        'pngset.c',
-        'pngtrans.c',
-        'pngusr.h',
-        'pngvcrd.c',
-        'pngwio.c',
-        'pngwrite.c',
-        'pngwtran.c',
-        'pngwutil.c',
-      ],
-      'direct_dependent_settings': {
-        'include_dirs': [
-          '.',
-        ],
-        'defines': [
-          'CHROME_PNG_WRITE_SUPPORT',
-          'PNG_USER_CONFIG',
-        ],
-      },
-      'export_dependent_settings': [
-        '../zlib/zlib.gyp:zlib',
-      ],
-      'conditions': [
-        ['OS!="win"', {'product_name': 'png'}],
-      ],
-    },
+    }],
   ],
 }