Nearly identical to bug 676860 except that it's a different header this time. I promise this is the last one; after this my plugin builds.

In /usr/include/rhythmbox/sources/rb-source.h (the shipped external plugin
headers that are provided after a `make install') we have the line

    #include <widgets/rb-search-entry.h>

This header is not shipped, because in widgets/Makefile.am we have

widgetinclude_HEADERS =                                 \
        rb-alert-dialog.h                               \
        rb-entry-view.h                                 \
        rb-property-view.h                              \
        rb-dialog.h                                     \
        rb-cell-renderer-pixbuf.h                       \
        rb-cell-renderer-rating.h                       \
        rb-rating.h                                     \
        rb-library-browser.h                            \
        rb-segmented-bar.h                              \
        rb-song-info.h                                  \
        rb-source-toolbar.h                             \
        rb-uri-dialog.h                                 \
        rb-fading-image.h                               \
        rb-object-property-editor.h


The widgetinclude_HEADERS is a list of files in the widgets/ directory that
are shipped in includedir. As you can see, rb-search-entry.h is not in the list.

Therefore, the following trivial C program will not compile:

#include <sources/rb-source.h>
int main() { return 0; }

with the command

gcc `pkg-config --cflags rhythmbox` -o test test.c


This prevents building third-party native plugins (C, C++, Vala, etc.) as add-ons in distros that ship recent Rhythmbox.

Thanks in advance.