selfcheck.sh: also run with system includes made available

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No description provided.

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This fails with

/usr/include/c++/15.1.1/ext/atomicity.h:39: syntax error: failed to evaluate #if condition, division/modulo by zero

The code in question:

#if __has_include(<sys/single_threaded.h>)
# include <sys/single_threaded.h>
#endif

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It now fails with

/usr/include/c++/v1/__availability:78: syntax error: failed to evaluate #if condition, undefined function-like macro invocation: __has_feature( ... )

or

/usr/include/c++/v1/__assert:29: syntax error: failed to evaluate #if condition, undefined function-like macro invocation: __has_builtin( ... )