sal_osl_process: fix "Parent and child environment not equal" for OSX <10.6

It fails with "assertion failed":
- Expression: compare_environments() [osl_process.cxx:370]
- Expression: compare_merged_environments(different_child_env_vars) [osl_process.cxx:428]

Just disable it (yet) for OSX <10.6 as it's already done for Windows.

Change-Id: I88e838481a25ee7e977512e33e23afa2b9d394ff
Reviewed-on: https://gerrit.libreoffice.org/10641
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Stephan Bergmann <sbergman@redhat.com>