It turns out that the "implicit declaration of function
{lgamma,native_tan} is invalid in C99" compiler warning results from
using an insufficiently up-to-date version of libclc, and would turn
into a linker error, if other problems did not prevent the compilation
from progressing that far. See here for details:https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=857710 I have filed a bug report against the Debian Blender package, to aggregate this bug and the two referenced above, which seem to be what is currently preventing GPU rendering from working in Blender. There are now solutions available for all three, which can and should be applied to the Debian archive; see the individual bug reports for details. https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=857718 -- Ian Bruce
