It is dangerous insofar as it pollutes an immense number of source
files. For example, utf8cpp's stuff uses relative namespace
names (e.g. `utf8::some_type`). If `using namespace libebml` is in
effect, then this clashes with `libebml::utf8` which is a typedef for
`unsigned char`, causing compiler errors.
gcc < 4.4 has a bug that is triggered by included a header with a
precompiled version more than once. Including a proxy header file that
wraps the actual precompiled one inside a #ifdef guard is fine, though.
