meson: meson.is_cross_build() returning false when "cross-compiling"

I’m developing on OS X, and trying to create a cross build that runs an actual GCC build (installed with homebrew), not the gcc-that’s-really-clang.

So I made a basic cross-file:

[binaries]
c = '/usr/local/opt/gcc/bin/gcc-9'
cpp = '/usr/local/opt/gcc/bin/g++-9'
ar = '/usr/local/opt/gcc/bin/gcc-ar-9'

[properties]
c_args = []
c_link_args = []
cpp_args = []
cpp_link_args = []

[host_machine]
system = 'darwin'
cpu_family = 'x86_64'
cpu = 'x86_64'
endian = 'little'

And invoke a cross build:

meson buildresults --cross-file=build/cross/gcc/osx-brew/gcc.txt

Meson identifies it as a cross build:

The Meson build system
Version: 0.51.999
Source dir: /Users/pjohnston/src/ea/embedded-framework
Build dir: /Users/pjohnston/src/ea/embedded-framework/buildresults
Build type: cross build
WARNING: Unknown options: "build.c_std, build.cpp_std"
Project name: Embedded Virtual Machine
Project version: 0.9
C compiler for the build machine: cc (clang 10.0.1 "Apple LLVM version 10.0.1 (clang-1001.0.46.4)")
C++ compiler for the build machine: c++ (clang 10.0.1 "Apple LLVM version 10.0.1 (clang-1001.0.46.4)")
C compiler for the host machine: /usr/local/opt/gcc/bin/gcc-9 (gcc 9.1.0 "gcc-9 (Homebrew GCC 9.1.0) 9.1.0")
C++ compiler for the host machine: /usr/local/opt/gcc/bin/g++-9 (gcc 9.1.0 "g++-9 (Homebrew GCC 9.1.0) 9.1.0")
Build machine cpu family: x86_64
Build machine cpu: x86_64

But this logic is saying we’re not a cross build:

if meson.is_cross_build()
	# ...
else
	message('Since this is not a cross build, target_*_args will match native_*_args')
	# ...
endif

When I run a cross build using ARM GCC, things get detected correctly. My hunch is that my minimal definition is missing something that meson is keying off of? So I’m still exploring that aspect.