Fix: do not use diagnostic pragma when GCC version is lower than 4.6.0

Fix: do not use diagnostic pragma when GCC version is lower than 4.6.0

Officially the diagnostic pragma are supported starting in 4.6. [1]
But they were present before 4.6 with limitation which we cannot
honour easily.

[1] https://gcc.gnu.org/gcc-4.6/changes.html

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>