Modified SConstruct to enable building of shared library

with hidden visibility on Mac OS X.  Patch from
Abdulla Kamar. http://codereview.chromium.org/475002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3460 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

SConstruct

@@ -143,6 +143,9 @@ LIBRARY_FLAGS = {
     },
     'os:macos': {
       'CCFLAGS':      ['-ansi', '-mmacosx-version-min=10.4'],
+      'library:shared': {
+        'CPPDEFINES': ['V8_SHARED']
+      }
     },
     'os:freebsd': {
       'CPPPATH' : ['/usr/local/include'],