build,cpp: Create forwarding headers in a gpgme++ folder

* lang/cpp/src/Makefile.am (copied_headers): New.
(build rule for all copied headers): New.
(BUILT_SOURCES): New.
(CLEANFILES): Add copied_headers.

The forwarding headers make it easier to use the headers from the Qt
bindings when building them against the built but not installed C++
bindings because we can always include the C++ headers with gpgme++/
prefix. This also helps to avoid including a C++ header with the same
name as a header in the Qt bindings (e.g. util.h).

• GnuPG-bug-id: T7110