Add missing default args for if toolchain can't be found

Small fix for an issue that cropped up with the new compilers for Conda.

If guess_toolchain fails, then we end up with a default GCCToolchain populated with reasonable values.
However, the GCCToolchain._cmdline() call requires that each of undefines and include_dirs be defined. I've added default values for the so_ext and o_ext for good measure.

In addition, I've slightly modified the test_missing_compilers such that we actually run a case with the default toolchain to ensure it's working correctly.