The default OS X 10.4 linker incorrectly searches for dependencies of
linked shared libraries only under the -isysroot location.  It fails to
find dependencies of linked shared libraries in cases such as the
ExportImport test.  It produces errors like:

 /usr/libexec/gcc/i686-apple-darwin8/4.0.1/ld: warning can't open dynamic library:
 libtestLib3Imp.dylib
 referenced from: /.../ExportImport/Root/lib/libtestLib3lib.1.2.dylib
 (checking for undefined symbols may be affected) (No such file or directory, errno = 2)
 /usr/libexec/gcc/i686-apple-darwin8/4.0.1/ld: Undefined symbols: _testLib3Imp
 referenced from libtestLib3lib expected to be defined in
 libtestLib3Imp.dylib

or with CMAKE_SKIP_RPATH off to enable install_name in the Export side:

 /usr/libexec/gcc/i686-apple-darwin8/4.0.1/ld: warning can't open dynamic library:
 /Developer/SDKs/MacOSX10.4u.sdk/.../ExportImport/Export/impl/libtestLib3Imp.dylib
 referenced from: /.../ExportImport/Export/libtestLib3lib.1.2.dylib
 (checking for undefined symbols may be affected) (No such file or directory, errno = 2)
 /usr/libexec/gcc/i686-apple-darwin8/4.0.1/ld: Undefined symbols:_testLib3Imp
 referenced from libtestLib3lib expected to be defined in
 /.../ExportImport/Export/impl/libtestLib3Imp.dylib

Note how "/Developer/SDKs/MacOSX10.4u.sdk" is prepended to the dependent
library path.

Commit 2cff26fa (Support linking to shared libs with dependent libs,
2008-01-31) and commit 82fcaebe (Pass dependent library search path to
linker on some platforms, 2008-02-01) worked around the problem by
defining platform variable CMAKE_LINK_DEPENDENT_LIBRARY_FILES.  It tells
CMake to link to dependent libraries explicitly by their path thus
telling the linker where to find them.

Unfortunately the workaround had the side effect of linking dependent
libraries and defeats most benefits of LINK_INTERFACE_LIBRARIES.
Fortunately OS X 10.5 and above do not need to find transitive
dependencies at all so we can avoid the workaround on Modern OS X.