[cairo] Make cairo-traces buildable on RHEL-7

[Matěj Cepl]

Hi,

I am working on packaging cairo-traces to Fedora/RHEL and I think I will
suggest some changes (packaging 500+MB of data is kind of bad,
considering Fedora mirrors etc., so I would rather prefer some kind of
script downloading data from somewhere).

However, before I can do any changes, I first have to make this package
build on RHEL-7 (or any other Fedora for that matter). I guess it is
because cairo-traces were originally written for older more lenient
version of GCC, but currently it shows missing #include of string.h and
missing linking of libm.

What do you think?

Matěj