[MacPorts] #54543: glib2-devel: projects using glib-genmarshal with C++ code break

[MacPorts]

#54543: glib2-devel: projects using glib-genmarshal with C++ code break
--------------------+-------------------------
 Reporter:  Ionic   |      Owner:
     Type:  defect  |     Status:  new
 Priority:  Normal  |  Milestone:
Component:  ports   |    Version:
 Keywords:          |       Port:  glib2-devel
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 `glib2` switched from a C-based `glib-genmarshal` tool to a new
 implementation in Python.

 Since then, projects using `glib-genmarshal` to generate code with
 parameters `--header --body` break if they compile the resulting file as
 C++ code, since the new implementation transparently maps `--header
 --body` to `--body --prototypes`, which does not wrap prototypes in
 `G_{BEGIN,END}_DECLS`. A separate header file is still wrapped correctly,
 leading to linking errors due to name mangling.

 The old tool did wrap the whole implementation file in
 `G_{BEGIN,END}_DECLS`, so that used to work.

 Upstream bug report: https://bugzilla.gnome.org/show_bug.cgi?id=785554

 Will attach a patch backporting that.

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Ticket URL: <https://trac.macports.org/ticket/54543>
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