Add BUILD_BULLET3_OPENCL option allowing PSP build

[jopadan]

For PSP compile use:

psp-cmake .  -DBUILD_EXTRAS=OFF -DBUILD_CPU_DEMOS=OFF -DBUILD_OPENGL3_DEMOS=OFF -DBUILD_BULLET2_DEMOS=OFF  -DBUILD_UNIT_TESTS=OFF -DBUILD_CLSOCKET=OFF -DBUILD_BULLET3_OPENCL=OFF --install-prefix=${PSPDEV}
make install