> fails with newer libc++:

Builds for me on a clean machine. gkoehler had a similar problem with
some ports, caused by an old /usr/include/c++/v1/setjmp.h.

> Making all in engine
> c++ -DHAVE_CONFIG_H -I. -I../../src   -I../../src -I/usr/local/include  -O2 -pipe -MT protocol.o -MD -MP -MF .deps/protocol.Tpo -c -o protocol.o protocol.cpp
> In file included from protocol.cpp:45:
> In file included from ./search.h:29:
> /usr/include/c++/v1/csetjmp:39:6: error: "If libc++ starts defining <setjmp.h>, the __has_include check should move to libc++'s <setjmp.h>"
>    39 | #    error "If libc++ starts defining <setjmp.h>, the __has_include check should move to libc++'s <setjmp.h>"
>       |      ^
> 1 error generated.