build fails on OmniOS using Clang 20 due to missing types

[infrastation]

$ uname -a
SunOS omnios-r151054 5.11 omnios-r151054-180b86b9dd i86pc i386 i86pc

# pkg install ooce/developer/clang-19 ooce/developer/clang-20

$ clang-19 --version
OmniOS/151054 clang version 19.1.7
Target: x86_64-pc-solaris2.11
Thread model: posix
InstalledDir: /opt/ooce/llvm-19/bin
Configuration file: /opt/ooce/llvm-19/bin/x86_64-pc-solaris2.11.cfg

$ CC=clang-19 ./configure && make -s
(works as expected)

$ clang-20 --version
OmniOS/151054 clang version 20.1.7
Target: x86_64-pc-solaris2.11
Thread model: posix
InstalledDir: /opt/ooce/llvm-20/bin
Configuration file: /opt/ooce/llvm-20/bin/x86_64-pc-solaris2.11.cfg

$ CC=clang-20 ./configure && make -s
[...]
In file included from fad-getad.c:49:
In file included from ./pcap-int.h:41:
In file included from ./pcap/pcap.h:130:
./pcap/bpf.h:93:9: error: unknown type name 'u_int'
   93 | typedef u_int bpf_u_int32;
      |         ^
./pcap/bpf.h:112:2: error: unknown type name 'u_int'
  112 |         u_int bf_len;
      |         ^
./pcap/bpf.h:240:2: error: unknown type name 'u_short'; did you mean 'short'?
  240 |         u_short code;
      |         ^~~~~~~
      |         short
./pcap/bpf.h:241:2: error: unknown type name 'u_char'; did you mean 'char'?
  241 |         u_char  jt;
      |         ^~~~~~
      |         char

(and so on)

It looks like with version 20 the system header is no longer included:

/usr/include/sys/types.h:675:typedef unsigned char      u_char;
/usr/include/sys/types.h:676:typedef unsigned short      u_short;
/usr/include/sys/types.h:677:typedef unsigned int       u_int;

The problem is not visible in the CI, which is currently using Clang 19. Clang 20 has been working as expected for a while on linux-aarch64. Clang 20 also works on Haiku.

[infrastation]

The problem with Clang 20 on OmniOS reproduces using both Autoconf and CMake.

[infrastation]

Commit 3f634fd documents Clang 20.1.2 as known-good, so it could be a regression in Clang 20.1 or the OmniOS package.