build: simplify v8's vendored zlib build

It only uses adler32() and only in one place that's arguably not very
performance critical so simplify the build by building just adler32.c
and nothing else.

tools/v8_gypfiles/v8.gyp

@@ -705,7 +705,7 @@
         'generate_bytecode_builtins_list',
         'run_torque',
         'v8_maybe_icu',
-        'zlib.gyp:zlib'
+        'v8_zlib',
       ],
       'includes': ['inspector.gypi'],
       'direct_dependent_settings': {
@@ -1599,5 +1599,16 @@
         'sources': ['<(SHARED_INTERMEDIATE_DIR)/debug-support.cc', ],
       },
     },  # postmortem-metadata
+
+    {
+      'target_name': 'v8_zlib',
+      'type': 'static_library',
+      'direct_dependent_settings': {
+        'include_dirs': [ '<(V8_ROOT)/third_party/zlib' ],
+      },
+      'defines': [ 'ZLIB_IMPLEMENTATION' ],
+      'include_dirs': [ '<(V8_ROOT)/third_party/zlib' ],
+      'sources': [ '<(V8_ROOT)/third_party/zlib/adler32.c' ],
+    },  # v8_zlib
   ],
 }