removed CMakeLists.txt from gpu/test folder to avoid precompiled header associated warnings under Ubuntu

fixed incorrect device id in multi_gpu sample (checked both multi GPU samples work correctly with new multi GPU API)

removed catch from GPU stereo_bp test as --gtest_catch_exceptions flags exists (but err msg isn't printed in case when there is no GPU)

Prefixed constants in flann with FLANN_ to prevent clashes with constants from other includes, closes bug #890

minor fixes and refactoring of GPU module

GPU: TargetArchs -> added FEATURE_SET prefix.

added assertion after all kernels calls

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