package/gcc: disable libcilkrts when there is no thread support

package/gcc: disable libcilkrts when there is no thread support

The libcilkrts configure script errors out with "Pthreads are required
to build libcilkrts" if the C library doesn't have thread support. To
fix that, we disable libcilkrts when thread support is not available.

This issue was not noticed until now, because we only regularly build
a no-thread toolchain for ARM, and libcilkrts was enabled on ARM only
starting in gcc 7.x.

This fixes the build of no-thread toolchains on architectures where
libcilkrts is supported, i.e x86/x86-64, ARM and Sparc.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>