xcode generator should append explicit settings in xcode_settings lists, rather than overwrite values previously set by the generator.

[GoogleCodeExporter]

** What steps will reproduce the problem?

Create a project with dependencies on two static libraries, defined in 
subprojects:
- subproject A adds a library to link_settings[libraries]
- subproject B adds a directory to 
link_settings[xcode_settings][LIBRARY_SEARCH_PATHS]

** What is the expected output? What do you see instead?

I expect the generated xcodeproj to have LIBRARY_SEARCH_PATHS set to both the 
directory of the library in subproject A and the path set by subproject B.

Instead, the setting from subproject B overwrites the path set for subproject 
A. 

**What version of the product are you using? On what operating system?

r1580, generating Xcode project on OS X.

**Please provide any additional information below.

Sample project attached. 

Original issue reported on code.google.com by jonw...@google.com on 5 Jun 2013 at 3:39

Attachments:

• sample.zip

[GoogleCodeExporter]

I mistyped the revision; it's actually r1589. 

Original comment by jonw...@google.com on 5 Jun 2013 at 3:42