VS 7.1 and below have 2 behaviors that make the cmComputeTargetDepends
result difficult to use for solution-level dependencies.  Update the
method cmGlobalVisualStudioGenerator::ComputeTargetDepends to document
the behaviors and work around them.  Commit 1a0c166a (Store direct
dependencies in solutions for VS >= 8, 2010-08-20) isolated VS >= 8 from
this computation so those versions should be unaffected.

This change removes the last use of cmTarget::GetLinkLibraries for
purposes other than backward compatibility with legacy interfaces
(export_library_dependencies, VS 6 custom .dsp templates).  Now the
cmComputeTargetDepends results are used for all generators so global
target dependency computation is fully centralized.