* gnu/packages/maths.scm (scotch): Update to 6.0.4.
[arguments]: Add -fPIC to CFLAGS.
* gnu/packages/patches/scotch-test-threading.patch: Adjust patch for a
new set of test fixes.
* gnu/packages/patches/pt-scotch-build-parallelism.patch: New patch.
* gnu-system.am (dist_patch_DATA): Add it.
