SConstruct new smoke_python_name() hangs compilation

When running scons to compile mongoDB from source, the smoke tests hang when calling smoke_python_name() at the second binary loop in the function. The compilation is then stale and won't go any further.

This patch fixes the problem by checking for the python binary in PATH instead of only relying on the exception handler.

NB : this patch also fixes user respect of FLAGS but you can ignore it if you want, this bug is fixed with the @@ -827,14 +830,16 @@ part only