On Darwin support custom install-id for the library.

Add DARWIN_INSTALL_NAME construction variable which can be set to
something else than the default absolute path.

Author: pavoljuhas

src/SConscript

@@ -83,10 +83,12 @@ Alias('lib', libobjcryst)
 prefix = env['prefix']
 
 # install-lib
-libinstall = Install(env['libdir'], libobjcryst)
+libinstall = env.Install(env['libdir'], libobjcryst)
 if env['PLATFORM'] == 'darwin':
+    # DARWIN_INSTALL_NAME can be pre-set in sconscript.local
+    env.SetDefault(DARWIN_INSTALL_NAME='$TARGET.abspath')
     env.AddPostAction(libinstall,
-            'install_name_tool -id $TARGET.abspath $TARGET')
+            'install_name_tool -id $DARWIN_INSTALL_NAME $TARGET')
 if env['PLATFORM'] == 'posix' and WhereIs('ldconfig'):
     opts = ''
     if os.getuid() != 0:  opts = '-n'