build: add DEVBUILD option

Add a cmake option DEVBUILD to enable compiler warnings and turn them into
errors.  The default is to build with the default warning level.

This avoids issues when building libipt with a new compiler until all
newly diagnosed warnings have been fixed.

Change-Id: Ie06c31a1df0784b12b37cddd1586bbdc757cb024
Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>