[CMake] Always use DYLD_LIBRARY_PATH on Apple in RootMacros

Closes #19327, where it was pretty clear from the logs that rootcling
was called with `LD_LIBRARY_PATH` on Apple:

```
cd /opt/local/var/macports/build/root6-317f8c0f/work/build/net/http && /opt/local/bin/cmake -E env LD_LIBRARY_PATH=/opt/local/var/macports/build/root6-317f8c0f/work/build/lib: ROOTIGNOREPREFIX=1 /opt/local/var/macports/build/root6-317f8c0f/work/build/bin/rootcling ..
dyld[90673]: Library not loaded: @rpath/libRIO.6.36.so
  Referenced from: <E1A17F5A-9228-3AB6-9D34-6EA7D65D2C99> /opt/local/var/macports/build/root6-317f8c0f/work/build/bin/rootcling
  Reason: tried: '/opt/local/lib/libgcc/libRIO.6.36.so' (no such file), '/System/Volumes/Preboot/Cryptexes/OS/opt/local/lib/libgcc/libRIO.6.36.so' (no such file), '/opt/local/lib/libRIO.6.36.so' (no such file), '/System/Volumes/Preboot/Cryptexes/OS/opt/local/lib/libRIO.6.36.so' (no such file), '/opt/local/var/macports/build/root6-317f8c0f/work/build/bin/../lib/root/libRIO.6.36.so' (no such file), '/opt/local/lib/libgcc/libRIO.6.36.so' (no such file), '/System/Volumes/Preboot/Cryptexes/OS/opt/local/lib/libgcc/libRIO.6.36.so' (no such file), '/opt/local/lib/libRIO.6.36.so' (no such file), '/System/Volumes/Preboot/Cryptexes/OS/opt/local/lib/libRIO.6.36.so' (no such file), '/opt/local/var/macports/build/root6-317f8c0f/work/build/bin/../lib/root/libRIO.6.36.so' (no such file)
```

This should have used `DYLD_LIBRARY_PATH`.

Actually, there is already a CMake variable `${ld_library_path}` with a
platform-dependent value, so we just need to use is consistently.

Author: guitargeek

cmake/modules/RootMacros.cmake

@@ -617,7 +617,7 @@ function(ROOT_GENERATE_DICTIONARY dictionary)
     if(MSVC AND CMAKE_ROOTTEST_DICT)
       set(command ${CMAKE_COMMAND} -E ${CMAKE_BINARY_DIR}/bin/rootcling_stage1.exe)
     else()
-      set(command ${CMAKE_COMMAND} -E env "LD_LIBRARY_PATH=${CMAKE_BINARY_DIR}/lib:$ENV{LD_LIBRARY_PATH}" $<TARGET_FILE:rootcling_stage1>)
+      set(command ${CMAKE_COMMAND} -E env "${ld_library_path}=${CMAKE_BINARY_DIR}/lib:$ENV{${ld_library_path}}" $<TARGET_FILE:rootcling_stage1>)
     endif()
     set(ROOTCINTDEP rconfigure)
     set(pcm_name)
@@ -626,17 +626,13 @@ function(ROOT_GENERATE_DICTIONARY dictionary)
       if(MSVC AND CMAKE_ROOTTEST_DICT)
         set(command ${CMAKE_COMMAND} -E env "ROOTIGNOREPREFIX=1" ${CMAKE_BINARY_DIR}/bin/rootcling.exe)
       else()
-        set(command ${CMAKE_COMMAND} -E env "LD_LIBRARY_PATH=${CMAKE_BINARY_DIR}/lib:$ENV{LD_LIBRARY_PATH}"
+        set(command ${CMAKE_COMMAND} -E env "${ld_library_path}=${CMAKE_BINARY_DIR}/lib:$ENV{${ld_library_path}}"
                     "ROOTIGNOREPREFIX=1" $<TARGET_FILE:rootcling> -rootbuild)
         # Modules need RConfigure.h copied into include/.
         set(ROOTCINTDEP rootcling rconfigure)
       endif()
     elseif(TARGET ROOT::rootcling)
-      if(APPLE)
-        set(command ${CMAKE_COMMAND} -E env "DYLD_LIBRARY_PATH=${ROOT_LIBRARY_DIR}:$ENV{DYLD_LIBRARY_PATH}" $<TARGET_FILE:ROOT::rootcling>)
-      else()
-        set(command ${CMAKE_COMMAND} -E env "LD_LIBRARY_PATH=${ROOT_LIBRARY_DIR}:$ENV{LD_LIBRARY_PATH}" $<TARGET_FILE:ROOT::rootcling>)
-      endif()
+      set(command ${CMAKE_COMMAND} -E env "${ld_library_path}=${ROOT_LIBRARY_DIR}:$ENV{${ld_library_path}}" $<TARGET_FILE:ROOT::rootcling>)
     else()
       set(command ${CMAKE_COMMAND} -E env rootcling)
     endif()
@@ -1873,7 +1869,7 @@ function(ROOT_ADD_PYUNITTESTS name)
   else()
     set(ROOT_ENV ROOTSYS=${ROOTSYS}
         PATH=${ROOTSYS}/bin:$ENV{PATH}
-        LD_LIBRARY_PATH=${ROOTSYS}/lib:$ENV{LD_LIBRARY_PATH}
+        ${ld_library_path}=${ROOTSYS}/lib:$ENV{${ld_library_path}}
         PYTHONPATH=${ROOTSYS}/lib:$ENV{PYTHONPATH})
   endif()
   string(REGEX REPLACE "[_]" "-" good_name "${name}")
@@ -1899,7 +1895,7 @@ function(ROOT_ADD_PYUNITTEST name file)
   else()
     set(ROOT_ENV ROOTSYS=${ROOTSYS}
         PATH=${ROOTSYS}/bin:$ENV{PATH}
-        LD_LIBRARY_PATH=${ROOTSYS}/lib:$ENV{LD_LIBRARY_PATH}
+        ${ld_library_path}=${ROOTSYS}/lib:$ENV{${ld_library_path}}
         PYTHONPATH=${ROOTSYS}/lib:$ENV{PYTHONPATH})
   endif()
   string(REGEX REPLACE "[_]" "-" good_name "${name}")