_Static_assert is a C11 keyword that maps to the C11 static_assert
macro. In C++ 2011 and newer, static_assert is the keyword so we map
_Static_assert to that to allow C11 headers to compile with C++ 2011.
For C++ prior to 2011, we fell back to one of many other ways to do
this, which conflicted with other things in the tree (which compile
on non C++ systems).

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So I'm not at all sure about this.
We can't safely define _Static_assert as a macro for C++ < C++11, since it's used in the base and since the base defines static_assert as _Static_assert there. In that case, it defines it as #define static_assert(...) _Static_assert(...). But no where else is it defined, leading to issues if the compiler doesn't secretly implement it.
So that leaves me unsure. It fixes the bug https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=276738 by two headers avoiding fighting over this.
But I find it deeply unsatisfying, to be honest.
For modern compilers, it will all work. For non-modern compilers, meh.
I worry that we're breaking older g++ compilers that build something needing our headers, which assume that they can use _Static_assert w/o issue. We do still support compiling USER CODE with older compilers (just not the world). It might help to have an example to argue against...