devel/ispc: fix build with libc++ >= 20
With libc++ 20 or higher devel/ispc fails to build, with errors similar
to:
/wrkdirs/usr/ports/devel/ispc/work/ispc-1.30.0/src/util.cpp:51:11: error: '__libcpp_verbose_abort' is missing exception specification 'noexcept'
51 | void std::__libcpp_verbose_abort(char const *format, ...)
| ^
| noexcept
/usr/include/c++/v1/__verbose_abort:24:28: note: previous declaration is here
24 | __printf__, 1, 2) void __libcpp_verbose_abort(const char* __format, ...) _NOEXCEPT;
| ^This is because ispc attemps to override __libcpp_verbose_abort(), which
is an internal libc++ function, but it misses a noexcept specification.
However, overriding __libcpp_verbose_abort() is only necessary on older
versions of macOS. Therefore, exclude the whole #if block if the
operating system is not macOS.
PR: 293163
Approved by: yuri (maintainer)
MFH: 2026Q1