Mention /etc/ld.so.preload to avoid LD_PRELOAD restrictions

[10110111]

LD_PRELOAD is ignored when you run setuid binaries, unset by sudo, while /etc/ld.so.preload is honored in all these cases. Also, in some cases other entities (like IDEs) may try to make a clean environment, which will also remove gtk3-nocsd from the preloaded libraries list.

So I think it'd be good to mention this approach in the README.

[10110111]

Actually, I've found that all this preloading business conflicts with some other libraries that expect to be loaded before all: e.g., libasan. Thus, both LD_PRELOAD and /etc/ld.so.preload are unacceptable to me.

But I've devised a better way, which I've implemented on my Ubuntu 19.04 system, and which works both system-wide (e.g. for root user), as well as without conflicts with libasan. Here's what I did:

origGLIB=$(ldd `which gtk-update-icon-cache` |
            sed -n 's@^\s*libglib-2.0.so.0 => \([^ ]\+\).*@\1@p')
libdir=${origGLIB%/libglib-2.0.so.0}/gtk3-nocsd
# Optional (if we want to disable CSDs unconditionally; I did)
sed -i 's@g_getenv ("GTK_CSD")@"0"@' gtk3-nocsd.c
# Add libglib-2.0.so.0 to NEEDED list, even though it's not statically used. We'll
# change reference to its actual soname to the name of symlink we'll create.
sed -i "s@.*-soname,libgtk3-no.*@\0 -Wl,-rpath,$libdir -Wl,--no-as-needed -lglib-2.0@"\
    Makefile
make
# Avoid patchelf --add-needed/--replace-needed: they'll likely corrupt the
# binary (at least in patchelf 0.10).
# Just rely on the fact that libgtk3-nocsd.so.0 only references the original glib-2.0
# library, so we can change _all_ the references to it to the symlink name.
sed -i 's@libglib-2.0.so.0@libglib2-orig.so@g' libgtk3-nocsd.so.0
sudo sh -c "mkdir -pv '$libdir' &&
            cp -vi libgtk3-nocsd.so.0 '$libdir'/libglib-2.0.so.0 &&
            ln -sv '$origGLIB' '$libdir'/libglib2-orig.so &&
            echo '$libdir' > /etc/ld.so.conf.d/00-gtk3-nocsd.conf &&
            ldconfig"