For some reasons, libcrypto would be built with the executable stack
flag set. I found out about this when Nginx failed to load the shared
library, because I was running it with MemoryDenyWriteExecute=true,
which does not permit executable stacks.

I am not sure why the stack ends up executable; the other shared
libraries which are part of LibreSSL do not have this flag set. You can
verify this with 'execstack -q'. Non-executable stacks should be the
default, and from checking some other files, that does appear to be the
case. The LibreSSL sources do not contain the string "execstack", so
I am not sure what causes the default to be overridden.

Adding '-z noexecstack' to the linker flags makes the linker unset the
flag. Now my Nginx can load the library, and so far I have not run into
other issues.

The flags to disable executable stacks are different for Clang and GCC,
and Clang is used on Darwin.

It is not needed on Darwin. [1] Thanks Matthew for explaining this.

[1]: #66454 (comment)

It turns out that libcrypto had an exectuable stack, because it linked
some objects without a .note.GNU-stack section. Compilers add this
section by default, but the objects produced from .S files did not
contain it. The .S files do include a directive to add the section, but
guarded behind an #ifdef HAVE_GNU_STACK. So define HAVE_GNU_STACK, to
ensure that all objects have a .note.GNU-stack section.

libressl: build libcrypto with noexecstack