add rpath for libraries specified with --link

[mcmehrtens]

I use Spack to manage my dependencies for Alamo and keep them separate from my system-installed dependencies which I need for other projects/software. Getting Alamo to find my Spack-installed dependencies during compile/link time was trivial using the --link flag, but the executable couldn't find the libpng dependency during runtime:

RUN    tests/AllenCahn
  ├ 2D-serial-level
  │      Running test............................................[FAIL]
  │      CMD   : ./bin/alamo-2d-clang++ tests/AllenCahn/input stop_time=10.0 plot_file=tests/AllenCahn/output_2025-11-25_18.59.40_matt-mbp-3.local_2D-serial-level
  │      STDOUT:
  │      STDERR: dyld[1690]: Library not loaded: @rpath/libpng16.16.dylib
  │      STDERR:   Referenced from: <BEABEF32-5F95-307E-AC2E-D79FB640690D> /Users/mcmehrtens/code/alamo/bin/alamo-2d-clang++
  │      STDERR:   Reason: no LC_RPATH's found
  │      STDERR:

Looking at the output from otool, I can confirm libpng is being referenced dynamically:

❯ otool -L bin/alamo-2d-clang++
bin/alamo-2d-clang++:
        @rpath/libpng16.16.dylib (compatibility version 16.0.0, current version 16.47.0)
        /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1356.0.0)
        /opt/homebrew/opt/gcc/lib/gcc/current/libgfortran.5.dylib (compatibility version 6.0.0, current version 6.0.0)
        /Users/mcmehrtens/tools/spack/opt/spack/darwin-m1/openmpi-5.0.9-xojzyn7jieqeynwgmsomyybgjtcal3iq/lib/libmpi.40.dylib (compatibility version 81.0.0, current version 81.7.0)
        /opt/homebrew/opt/llvm/lib/c++/libc++.1.dylib (compatibility version 1.0.0, current version 1.0.0)

But, there doesn't appear to be a path set that would tell the executable where to find my Spack-installed libpng library:

$ otool -l ./bin/alamo-2d-clang++ | grep -A2 LC_RPATH

One fix (as implemented in this PR) would be to add any libraries that were added with --link to the rpath using -Wl,-rpath,/path/to/library. If you do this, then we correctly see the LC_RPATH with otool:

$ otool -l ./bin/alamo-2d-clang++ | grep -A2 LC_RPATH
          cmd LC_RPATH
      cmdsize 48
         path /opt/homebrew/opt/llvm/lib/c++ (offset 12)
--
          cmd LC_RPATH
      cmdsize 56
         path /opt/homebrew/opt/gcc/lib/gcc/current (offset 12)
--
          cmd LC_RPATH
      cmdsize 120
         path /Users/mcmehrtens/tools/spack/opt/spack/darwin-m1/libpng-1.6.47-s4qucbbozbdwq7botwwkeeiknhcn7wzz/lib (offset 12)
--
          cmd LC_RPATH
      cmdsize 112
         path /Users/mcmehrtens/tools/spack/opt/spack/darwin-m1/fftw-3.3.10-6ogec24drajjfbxswvxtrie2ad4sorlw/lib (offset 12)

and we no longer get the error about not being able to find the libpng library:

RUN    tests/AllenCahn
  ├ 2D-serial-level
  │      Running test............................................[PASS] (2.32s)
  │      Checking result.........................................[PASS]