fdo#72394 Don't endlessly expand $(share_subdir_name) into itself

The introduction of $(share_subdir_name) expanding to "share" in
c6a73009747814513ab5a7277c211449c7378870 "Use subfolder names from
<config_folders.h>" shows that
SubstitutePathVariables::impl_reSubstituteVariables is fundamentally flawed in
that it allows reducing variable occurrences even in variable names, so that it
will expand an occurrence of "share" into an endless recursion of
"$(share_subdir_name)" -> "$($(share_subdir_name)_subdir_name)" -> ...

Adding $(share_subdir_name) to the list of variables that can only replace a
complete path segment (delimited by "/"), from which it had erroneously been
missing, "fixes" this problem for now.

But the code is still wrong in at least two respects:
* SubstitutePathVariables::impl_reSubstituteVariables arguably needs to be made
  more robust, to never reduce variable occurrences in variable names.
* The compile-time variable LIBO_SHARE_FOLDER should not end up as a runtime
  framework path variable, esp. since accidentally re-substituting it for
  "share" segments in unrelated URLs like <file:///export/share/for-all> does
  not make sense.  ac4e19f9085dbd0103c7336a5318aa1e55b3e3e0 "fdo#68552: Don't
  (attempt to) do run-time expansion of build-time parameters" had already
  attempted a fix for that, but it had to be reverted again with
  791a8b96f754798192875da287c84f8cfa4e533e because it "Unfortunately does not
  work if BUILDDIR is different from SRCDIR."

Change-Id: If214c179c0068fbaa475c1c9cac804d6a1dbb2dc