Avoid duplicate RPATH entries

Teach cmComputeLinkInformation::GetRPath to avoid adding the same
directory to the output runtime path more than once.
AIX-GNU: Put implicit link directories in runtime libpath (#13909)

The GNU compiler front-ends on AIX invoke the linker with flags of the
form "-L/path/to/gnu/runtime/lib" to tell ld where to find the language
runtime libraries.  They depend on the default libpath behavior
documented in "man ld" to add the -L paths also to the runtime libpath
so the dynamic loader can find the language runtime libraries.  This
differs from platforms whose linkers have distinct -rpath flags that
non-system compilers can use to tell the dynamic loader where to find
their language runtime libraries.

Since commit 96fd5909 (Implement linking with paths to library files,
2008-01-22) CMake always passes "-Wl,-blibpath:" followed by any
project-defined RPATH plus "/usr/lib:/lib" in order to explicitly set
the runtime libpath and avoid getting all the project -L paths in the
runtime libpath.  The explicit libpath prevents the GNU compiler runtime
library -L paths from being placed in the libpath and then the dynamic
loader fails to find the language runtime libraries.

CMake already detects the implicit link directories for each language
since commit 07ea19ad (Implicit link info for C, CXX, and Fortran,
2009-07-23).  Add the implicit link directories to the explicit runtime
libpath for GNU compilers on AIX to fix this use case.
Merge topic 'LINK_LIBRARIES-property' into rpath-use-implicit-link-dirs

Resolve a logical conflict in the signature of cmTarget::GetLinkClosure.
Merge topic 'rpath-use-implicit-link-dirs' into next

95a9c80 Merge topic 'LINK_LIBRARIES-property' into rpath-use-implicit-link-dirs
baa33ac AIX-GNU: Put implicit link directories in runtime libpath (#13909)
171b099 Avoid duplicate RPATH entries