I think this post answers your question, there is a bug in xcode, according to the signature, the post is by a developer who works on the xcode product. The reply to it shows a solution that has worked for me, which is:
- explicitly set the location of the static library to "Relative to Build Products"
with a good text editor, edit the project.pbxproj inside the your project e.g. MyProject.xcodeproj/project.pbxproj and search for a line like the one below containing your library name. Verify that for the path= entry there is no path, and only the filename of your library file. It's nasty to have to do this, but I have found that xcode does not get it right 100% of the time.
{isa = PBXFileReference; explicitFileType = archive.ar; includeInIndex = 0; path = libMyLibrary.a; sourceTree = BUILT_PRODUCTS_DIR; };