The existing tst_qgetputenv shows that qputenv with an empty value
doesn't lead to the same result on Windows and on Unix, and there was
no way to fully delete an env var on Unix (which is needed for some
env vars where not-set and empty are different, such as TZ,
see `man tzset`).
This is also why qglobal has qEnvironmentVariableIsSet() vs
qEnvironmentVariableIsEmpty(), on the getter side.
Qt4's ifdefs around unsetenv in qapplication_x11.cpp show that this is
needed within Qt too (although this particular startup notification code
has to be re-imported into Qt5 still).
Change-Id: I631c8cddbcf933d4b9008f11aefc59f5a3c7c866
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
574e5cf9c5