Use __PRETTY_FUNCTION__ in Clang as well
Under MSC, __FUNCSIG__ is closer to __PRETTY_FUNCTION__ than __FUNCTION__
Add comment
Addres signed/unsigned mismatch warning
Provide default constructor for abc::external_buffer_t

Clang says it’s necessary in order to construct abc::external_buffer,
which is const.
Add support for Clang as GNU LD driver