osmesa-src fails to find nir symbols - macos 10.14.5

[mrwonko]

I'm currently trying to build servo, but I get this error during the osmesa-src build:

ld: warning: object file (spirv/spirv2nir-spirv2nir.o) was built for newer OSX version (10.14) than being linked (10.10)
ld: warning: ignoring file nir/.libs/libnir.a, file was built for archive which is not the architecture being linked (x86_64): nir/.libs/libnir.a
ld: warning: ignoring file ../../src/util/.libs/libmesautil.a, file was built for archive which is not the architecture being linked (x86_64): ../../src/util/.libs/libmesautil.a
Undefined symbols for architecture x86_64:
  "_nir_print_shader", referenced from:
      _main in spirv2nir-spirv2nir.o
  "_spirv_to_nir", referenced from:
      _main in spirv2nir-spirv2nir.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

I've checked, and the missing symbols seem to be from the spirv/spirv2nir-spirv2nir.o that's being ignored. For some reason only parts of the code seem to be compiled targetting 10.10.

[jdm]

I suspect that's caused by https://github.com/servo/servo/blob/master/python/servo/build_commands.py#L319. Possibly adding that to CFLAGS and rebuilding the whole library would improve matters?