This is with latest rhythmbox from git, -Werror produces this:

/bin/sh ../../libtool  --tag=CC --tag=disable-static  --mode=compile gcc -DHAVE_CONFIG_H -I. -I../.. -DGNOMELOCALEDIR=\""/usr/share/locale"\" -DG_LOG_DOMAIN=\"Rhythmbox\" -I../.. -I../../lib -I../../metadata -I../../player -I../../rhythmdb -I../../widgets -I../../sources -I../../iradio -I../../podcast -I../../remote -I../../remote -I../../lib -I../../plugins -I../../shell -I../../plugins -DPIXMAP_DIR=\""/usr/share/pixmaps"\" -DSHARE_DIR=\"/usr/share/rhythmbox\" -DDATADIR=\""/usr/share"\" -D_REENTRANT -I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include   -DORBIT2=1 -pthread -I/usr/include/gtk-2.0 -I/usr/lib/gtk-2.0/include -I/usr/include/atk-1.0 -I/usr/include/cairo -I/usr/include/pango-1.0 -I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include -I/usr/include/pixman-1 -I/usr/include/freetype2 -I/usr/include/libpng12 -I/usr/include/gio-unix-2.0/ -I/usr/include/gnome-media -I/usr/include/gconf/2 -I/usr/include/libglade-2.0 -I/usr/include/orbit-2.0 -I/usr/include/dbus-1.0 -I/usr/lib/dbus-1.0/include -I/usr/include/libxml2 -I/usr/include/libsoup-2.4 -I/usr/include/libsoup-gnome-2.4   -pthread -I/usr/include/gstreamer-0.10 -I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include -I/usr/include/libxml2   -Wcomment -Wformat -Wnonnull -Wimplicit-int -Wimplicit -Wmain -Wmissing-braces -Wparentheses -Wsequence-point -Wreturn-type -Wswitch -Wtrigraphs -Wunused-function -Wunused-label -Wunused-value -Wchar-subscripts -Wmissing-declarations -Wmissing-prototypes -Wnested-externs -Wpointer-arith -Wcast-align -Wall -Werror -std=gnu89 -I/usr/include/dbus-1.0 -I/usr/lib/dbus-1.0/include -I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include   -DDBUS_API_SUBJECT_TO_CHANGE -D_XOPEN_SOURCE -D_BSD_SOURCE -I/usr/include/gnome-keyring-1 -I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include      -O2 -g -pipe  -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector --param=ssp-buffer-size=4 -fomit-frame-pointer -march=i586 -mtune=generic -fasynchronous-unwind-tables  -I/usr/include -MT libdaap_la-rb-daap-connection.lo -MD -MP -MF .deps/libdaap_la-rb-daap-connection.Tpo -c -o libdaap_la-rb-daap-connection.lo `test -f 'rb-daap-connection.c' || echo './'`rb-daap-connection.c
libtool: compile:  gcc -DHAVE_CONFIG_H -I. -I../.. -DGNOMELOCALEDIR=\"/usr/share/locale\" -DG_LOG_DOMAIN=\"Rhythmbox\" -I../.. -I../../lib -I../../metadata -I../../player -I../../rhythmdb -I../../widgets -I../../sources -I../../iradio -I../../podcast -I../../remote -I../../remote -I../../lib -I../../plugins -I../../shell -I../../plugins -DPIXMAP_DIR=\"/usr/share/pixmaps\" -DSHARE_DIR=\"/usr/share/rhythmbox\" -DDATADIR=\"/usr/share\" -D_REENTRANT -I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include -DORBIT2=1 -pthread -I/usr/include/gtk-2.0 -I/usr/lib/gtk-2.0/include -I/usr/include/atk-1.0 -I/usr/include/cairo -I/usr/include/pango-1.0 -I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include -I/usr/include/pixman-1 -I/usr/include/freetype2 -I/usr/include/libpng12 -I/usr/include/gio-unix-2.0/ -I/usr/include/gnome-media -I/usr/include/gconf/2 -I/usr/include/libglade-2.0 -I/usr/include/orbit-2.0 -I/usr/include/dbus-1.0 -I/usr/lib/dbus-1.0/include -I/usr/include/libxml2 -I/usr/include/libsoup-2.4 -I/usr/include/libsoup-gnome-2.4 -pthread -I/usr/include/gstreamer-0.10 -I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include -I/usr/include/libxml2 -Wcomment -Wformat -Wnonnull -Wimplicit-int -Wimplicit -Wmain -Wmissing-braces -Wparentheses -Wsequence-point -Wreturn-type -Wswitch -Wtrigraphs -Wunused-function -Wunused-label -Wunused-value -Wchar-subscripts -Wmissing-declarations -Wmissing-prototypes -Wnested-externs -Wpointer-arith -Wcast-align -Wall -Werror -std=gnu89 -I/usr/include/dbus-1.0 -I/usr/lib/dbus-1.0/include -I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include -DDBUS_API_SUBJECT_TO_CHANGE -D_XOPEN_SOURCE -D_BSD_SOURCE -I/usr/include/gnome-keyring-1 -I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include -O2 -g -pipe -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector --param=ssp-buffer-size=4 -fomit-frame-pointer -march=i586 -mtune=generic -fasynchronous-unwind-tables -I/usr/include -MT libdaap_la-rb-daap-connection.lo -MD -MP -MF .deps/libdaap_la-rb-daap-connection.Tpo -c rb-daap-connection.c  -fPIC -DPIC -o .libs/libdaap_la-rb-daap-connection.o
cc1: warnings being treated as errors
rb-daap-connection.c: In function 'T.162':
/usr/include/glib-2.0/glib/gthread.h:344: error: dereferencing pointer '_SOUP_METHOD_GET.50' does break strict-aliasing rules
rb-daap-connection.c:346: note: initialized from here


I'm using glib 2.21.4 and libsoup 2.27.5 on Mandriva Cooker.