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: 7110

lang/cpp/src/Makefile.am

@@ -88,6 +88,14 @@ else
 libsuffix=.so
 endif
 
+copied_headers = $(gpgmepp_headers:%=gpgme++/%) $(interface_headers:%=gpgme++/%)
+
+$(copied_headers): Makefile.am
+	mkdir -p $(builddir)/gpgme++/interfaces
+	echo -n "#include \"$(srcdir)" > "$@"
+	echo -n "$@" | sed "s/gpgme++//" >> "$@"
+	echo "\"" >> "$@"
+
 if HAVE_W32_SYSTEM
 GpgmeppConfig.cmake: GpgmeppConfig-w32.cmake.in
 	sed -e 's|[@]resolved_bindir@|$(bindir)|g' < "$<" | \
@@ -115,5 +123,8 @@ install-data-local: install-cmake-files
 
 uninstall-local: uninstall-cmake-files
 
+BUILT_SOURCES = $(copied_headers)
+
 CLEANFILES = GpgmeppConfig.cmake GpgmeppConfigVersion.cmake \
-             gpgmepp_version.h GpgmeppConfig.cmake.in
+             gpgmepp_version.h GpgmeppConfig.cmake.in \
+             $(copied_headers)