Hack for making build with CMake 2.8 and Ninja

CMake 2.8 creates faulty build.ninja files if a path contains spaces.
This leads to an error when ninja is invoked later on.

Excert:
CMake 2.8.12.2 (wrong):
  DEP_FILE = debuggers/gdb/unittests/path with
space/CMakeFiles/spacedebugee.dir/spacedebugee.cpp.o.d
CMake 3.0+:
  DEP_FILE = debuggers/gdb/unittests/path\ with\
space/CMakeFiles/spacedebugee.dir/spacedebugee.cpp.o.d