configure: add libpng to client libs

configure: add libpng to client libs

http://lists.freedesktop.org/archives/wayland-devel/2012-January/001975.html
reports a linking problem:

/usr/bin/ld: libtoytoolkit.a(cairo-util.o): undefined reference to
symbol 'png_set_filler@@PNG12_0'
/usr/bin/ld: note: 'png_set_filler@@PNG12_0' is defined in DSO
/usr/lib/i386-linux-gnu/libpng12.so.0 so try adding it to the linker command line
/usr/lib/i386-linux-gnu/libpng12.so.0: could not read symbols: Invalid
operation
collect2: ld returned 1 exit status
make[3]: [weston-terminal] Error 1 (ignored)

A similar problem is diagnosed here:
http://lists.fedoraproject.org/pipermail/devel/2010-March/133601.html

As some distros are shipping linkers, that do not resolve symbols from
implicitly linked libraries, check and link libpng explicitly.

Cc: nerdopolis <bluescreen_avenger@verizon.net>
Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>