Updated CMakeLists.txt to fix ValveSoftware/vogl/issues/16;

Updated CMakeLists.txt to fix ValveSoftware/vogl/issues/16;

src/CMakeLists.txt:
 (1) Added cmake variable HAS_UPDATED_LIBUNWIND;
 (2) Added message explaining what HAS_UPDATED_LIBUNWIND does;
 (3) Moved add_subdirectory(libbacktrace), add_subdirectory(libbacktrace_test);
     below other add_subdirectory(...) commands;

src/libbacktrace/CMakeLists.txt:
 (1) add_definitions(-DCMAKE_HAS_UPDATED_LIBUNWIND) if HAS_UPDATED_LIBUNWIND=true

src/libbacktrace/btrace.cpp:
 (1) added ifdef check CMAKE_HAS_UPDATED_LIBUNWIND