[xml/sgml-pkgs] Bug#801989: libxslt must be linked with libm

[Max Kellermann]

Package: libxslt1-dev
Version: 1.1.28-2+b2

Various programs fail to link unless they just happen to specify
"-lm":

 /usr/lib/gcc/x86_64-linux-gnu/5/../../../x86_64-linux-gnu/libxslt.so: error: undefined reference to 'pow'
 /usr/lib/gcc/x86_64-linux-gnu/5/../../../x86_64-linux-gnu/libxslt.so: error: undefined reference to 'floor'
 /usr/lib/gcc/x86_64-linux-gnu/5/../../../x86_64-linux-gnu/libxslt.so: error: undefined reference to 'fmod'