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Sorry if there is something unclear.
Redefine means: Set a value which is required to satisfy
ifdef-statements in the header files. As you see in the
definitions, given in the comment, the value for _NO_XOPEN4
is set as a result of some dependencies automatically
defined. A redefinition of _SGIAPI in the socketmodule.c is
not sufficient alone. Therefore to satisfy all requirements
in the header files, you should set the values for
_NO_XOPEN4 and _NO_XOPEN5 also manually.
Howto do in socketmodule.c?
...
#define _SGIAPI 1
#define _NO_XOPEN4 1
#define _NO_XOPEN5 1
...
If you are familiar with regular expressions, then you will
know that _NO_XOPEN[45] is a shorthand for _NO_XOPEN4,
_NO_XOPEN5.

Sorry if there was something wrong in my comment.