Xcode: Add projectDir parameter to GenerateBuildCommand

Finish change in commit a5631ce9 (Add projectDir parameter to
GenerateBuildCommand, 2013-03-21) for Xcode generator.
Fix CTest.BuildCommand.ProjectInSubdir on non-Windows

This test uses the test added by commit 5203460c (VS: Add test for
building MSBuild project in subdir, 2013-01-03) so do not use
Windows-specific commands.
Merge topic 'msbuild-targets-in-subdirs' into next

6fb2e24 Fix CTest.BuildCommand.ProjectInSubdir on non-Windows
57bab2d Xcode: Add projectDir parameter to GenerateBuildCommand