package/supertux: fix build with RELRO

package/supertux: fix build with RELRO

Disable static building of external/squirrel to fix the following build
failure with RELRO:

/home/peko/autobuild/instance-0/output-1/host/opt/ext-toolchain/bin/../lib/gcc/x86_64-buildroot-linux-musl/8.3.0/../../../../x86_64-buildroot-linux-musl/bin/ld: CMakeFiles/sq_static.dir/sq.c.o: relocation R_X86_64_32 against `.rodata.str1.8' can not be used when making a PIE object; recompile with -fPIC
/home/peko/autobuild/instance-0/output-1/host/opt/ext-toolchain/bin/../lib/gcc/x86_64-buildroot-linux-musl/8.3.0/../../../../x86_64-buildroot-linux-musl/bin/ld: final link failed: nonrepresentable section on output
collect2: error: ld returned 1 exit status

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

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>