make GATE_COMPILE_DEBUG_MESSAGES cmake compatible

[m-dupont]

We should make cmake aware of GATE_COMPILE_DEBUG_MESSAGES. Two ways to do this :

• set the GATE_COMPILE_DEBUG_MESSAGES when -DCMAKE_BUILD_TYPE=Debug is used.
• or add a new config option for cmake.

what do you think ? I have a preference for the first.

[dsarrut]

Hi, yes the first one is probably simpler.