Fix compilation for 3.0 branch (>= 3.0.39)

The isolate_mode_t type that appeared in 3.2 was backported to 3.0.39 so
the version check must be fixed. It was not backported to the 3.1 branch
though, so it must be excluded.

Signed-off-by: Simon Marchi <simon.marchi@polymtl.ca>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>