Not yet implemented in the code, but my idea is that any functionality
that modifies the system-wide installation will go away in this case.

Automatically set if --disable-externsions, or if building a sandboxed
LO for OS X.

Should probably be set automatically also when just building a signed
(but not necesssarily sandboxed) LO for OS X? Surely installing a
system-wide extension should count as tampering with the
app. Especially if we can make also extension (scripts) be signed (by
locating them in the Resources folder?)

Change-Id: Id654bfaa6331535a66eae1bc6531a756085a3f06