don't build gcc torture tests -O1 by default
prepare CMake environment for -O0 build

Use Debug as the CMAKE_BUILD_TYPE to set compile flags to -O0 and skip torture
tests that fail to compile. (The tests fail on the host too.)
mute conversion warnings for gcc torture tests
Merge pull request #2 from alexjordan/opt-levels

prepared CMake environment for -O0 building & testing