C++ compiler differences between istio/proxy and istio/envoy

I’m having an issue building istio/proxy on ppc64le. I can run bazel build //source/exe:envoy-static from istio/envoy, but from istio/proxy, I run into an error during a cmake step building zlib:

./zconf.h:249:14: fatal error: ‘stddef.h’ file not found
# include <stddef.h>

from the cmake log:

Manually-specified variables were not used by the project: CMAKE_CXX_COMPILER_FORCED