headers: Implement _set_com_error_handler and _com_raise_error in comdef.h

These could also possibly be made non-inline if we'd be providing
the comsupp library. However, that could break existing code that
calls _com_issue_error (implicitly via _com_ptr_t in comip.h)
and don't currently explicitly link in any comsupp library (which
doesn't exist currently either).

In the corresponding MSVC setups, the comsupp library is linked
in automatically via '#pragma comment(lib, "comsupp.lib")' in
the comdef.h header.

Signed-off-by: Martin Storsjö <martin@martin.st>