I am confused how arts and xorg are related.  Is libgthread installed
from a pkgsrc package, or is it part of the system (I don't know linux
this intimately).  If it is a system thread library, there's probably
something wrong with the pthread support.  This is not necessarily a
pkgsrc problem, as on Linux pthread.buildlink3.mk uses -pthread, which
leaves it up to the system compiler to choose the library.  Is arts
supposed to be linked with libgthread?  You may want to try setting
PTHREAD_CFLAGS & PTHREAD_LDFLAGS to empty (or set PTHREAD_CFLAGS to
whatever it should be) in the pkg Makefile and setting PTHREAD_LIBS to
-lpthread.

Let me know how it goes,

--Blair

Also, you may wish to try modular Xorg instead of Xorg 6.9.0.

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"The frivolity and boredom which unsettle the established order, the
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face of the whole is cut short by a sunburst which, in one flash,
illuminates the features of the new world."  --G.W.F. Hegel,
_Phenomenology of Spirit_ 5:11