package/links: add optional support for librsvg

package/links: add optional support for librsvg

When librsvg was compiled before, links will use it as optional
dependency, along with other gtk libraries:

$ output/host/usr/bin/x86_64-linux-readelf -a output/target/usr/bin/links | grep NEEDED | sort
 0x0000000000000001 (NEEDED)             Shared library: [libbz2.so.1.0]
 0x0000000000000001 (NEEDED)             Shared library: [libcairo.so.2]
 0x0000000000000001 (NEEDED)             Shared library: [libcrypto.so.1.0.0]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libevent-2.0.so.5]
 0x0000000000000001 (NEEDED)             Shared library: [libgdk_pixbuf-2.0.so.0]
 0x0000000000000001 (NEEDED)             Shared library: [libgio-2.0.so.0]
 0x0000000000000001 (NEEDED)             Shared library: [libglib-2.0.so.0]
 0x0000000000000001 (NEEDED)             Shared library: [libgobject-2.0.so.0]
 0x0000000000000001 (NEEDED)             Shared library: [libgpm.so.2]
 0x0000000000000001 (NEEDED)             Shared library: [libintl.so.8]
 0x0000000000000001 (NEEDED)             Shared library: [libjpeg.so.8]
 0x0000000000000001 (NEEDED)             Shared library: [liblzma.so.5]
 0x0000000000000001 (NEEDED)             Shared library: [libm.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libpng16.so.16]
 0x0000000000000001 (NEEDED)             Shared library: [libpthread.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [librsvg-2.so.2]
 0x0000000000000001 (NEEDED)             Shared library: [libssl.so.1.0.0]
 0x0000000000000001 (NEEDED)             Shared library: [libtiff.so.5]
 0x0000000000000001 (NEEDED)             Shared library: [libX11.so.6]
 0x0000000000000001 (NEEDED)             Shared library: [libz.so.1]

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>