Specify CMAKE_INSTALL_RPATH for macOS; use -rpath LDFLAGS for tests

[mwtoews]

The primary change in this PR applies to macOS users, which allows the library and utilities to be relocatable, using a relative path to the library via ../lib. There was a "TO DO: cmake 2.8.12" note in src/CMakeLists.txt that provided accurate guidance on this. Before this change, a user might see (e.g.):

$ /tmp/proj_cmake_install_renamed/subdir/bin/projsync
dyld: Library not loaded: @rpath/libproj.22.dylib
  Referenced from: /tmp/proj_cmake_install_renamed/subdir/bin/projsync
  Reason: image not found

the RPATH setting in CMake was already used for Linux so that the library and utilities are relocatable.

Other changes:

• Don't use LD_LIBRARY_PATH or DYLD_LIBRARY_PATH variables
• Quieter conda install messages, and install jsonschema required for testing

Other changes within test/postinstall:

• Rename test_ldd to test_libpath to be more generic
• For macOS, use otool -l to look for a matching LC_RPATH entry
• When necessary, use export LDFLAGS="${LDFLAGS} -Wl,-rpath,$(pkg-config proj --variable=libdir)" to "bake" the correct RPATH into objects (not needed for test_cmake or static builds)

[mwtoews]

I would appreciate any comment from mac users, as I don't have access to one. These tests were developed thru non-interactive runs on GitHub Actions.

Perhaps @hobu recalls changes to src/CMakeLists.txt in fab753b from 7 years ago? Do the changes look appropriate?