graphics/darktable: Update to 1.6.9

Use Clang 3.7 from Ports to enable OpenMP. This brings a major
performance boost for people using FreeBSD 10.x and -CURRENT.

patch-src_CMakeLists.txt is a new patch to disable -ffast-math when
Clang is used. Without this, isnan() always returns false, even if
-fno-finite-math-only is set. According to clang --help, this flag
provides no optimization anyway so we don't loose anything. darktable
2.0, which is almost ready, needs this patch too otherwise it crashes
during startup because of the isnan() misbehavior. The patch is not
committed upstream yet because I would prefer to understand what's
wrong.

Reviewed by: kwm
Approved by: kwm
Differential Revision: https://reviews.freebsd.org/D3922