Log Message:
Mixing _XOPEN_SOURCE=500 and _XOPEN_SOURCE=600 in the same package
is asking for trouble. On solaris these options are mutually exclusive
because 600 needs C99 and 500 is not allowed to use C99.

I lowered the requirement to _XOPEN_SOURCE=500 and the build succeeded.

While here I'll note that PR pkg/42897 (netbpm link error on Solaris 10)
should now be fixed by libpng-1.2.41nb1.