package/supertux: build squirrel builtin library with fPIC

package/supertux: build squirrel builtin library with fPIC

Ensure that squirrel is compiled with -fPIC to allow linking the static
libraries with dynamically linked programs. This is not a requirement
for most architectures but is mandatory for ARM.

Fixes:
http://autobuild.buildroot.org/results/46e8f5e622ce450a89bc6d70f4bfd38182557901
http://autobuild.buildroot.org/results/a43720492d817e4555d728546da9114e3ccba952

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>