[cmake] do not check none-existing variables

Then checking ROOTSYS, do not try to modify
none-existing variables. Like LD_LIBRARY_PATH on windows.
Also no need to change ROOTSYS if it set correctly

Author: linev

CMakeLists.txt

@@ -57,19 +57,26 @@ set(DIR_PERMISSIONS DIRECTORY_PERMISSIONS OWNER_READ OWNER_WRITE OWNER_EXECUTE G
 # Before setting ROOTSYS, make sure that the environment isn't polluted by a different
 # ROOT build. This is significant e,g. for roottest, which will otherwise have libraries
 # of a different ROOT build available / visible / reachable.
-if(NOT $ENV{ROOTSYS} STREQUAL "")
+if(NOT $ENV{ROOTSYS} STREQUAL "" AND NOT $ENV{ROOTSYS} STREQUAL "${CMAKE_BINARY_DIR}")
   function(strip_path ENV_VAR _path)
-    string(REPLACE ":${_path}:" ":" _temp_envvar $ENV{${ENV_VAR}})
-    string(REGEX REPLACE "^${_path}:" "" _temp_envvar ${_temp_envvar})
-    string(REGEX REPLACE ":${_path}$" "" _temp_envvar ${_temp_envvar})
-    if (_temp_envvar STREQUAL _path)
-      set(ENV{ENV_VAR} "")
-    else()
-      set(ENV{ENV_VAR} "${_temp_envvar}")
+    if(NOT $ENV{${ENV_VAR}} STREQUAL "")
+      string(REPLACE ":${_path}:" ":" _temp_envvar $ENV{${ENV_VAR}})
+      string(REGEX REPLACE "^${_path}:" "" _temp_envvar ${_temp_envvar})
+      string(REGEX REPLACE ":${_path}$" "" _temp_envvar ${_temp_envvar})
+      if (_temp_envvar STREQUAL _path)
+        set(ENV{ENV_VAR} "")
+      else()
+        set(ENV{ENV_VAR} "${_temp_envvar}")
+      endif()
     endif()
   endfunction()
   strip_path("PATH" "$ENV{ROOTSYS}/bin")
-  strip_path("LD_LIBRARY_PATH" "$ENV{ROOTSYS}/lib")
+  if(not WIN32)
+    strip_path("LD_LIBRARY_PATH" "$ENV{ROOTSYS}/lib")
+  endif()
+  if(APPLE)
+    strip_path("DYLD_LIBRARY_PATH" "$ENV{ROOTSYS}/lib")
+  endif()
   strip_path("PYTHONPATH" "$ENV{ROOTSYS}/lib")
   # if we leave the ':' it will result in an empty entry in the CMAKE_PREFIX_PATH
   # which will interpreted as the current directory.