Stdint.h: No such file or directory when compiling

I am trying to compile Redox on MacOS.
I get the following error and I’ve failed to figure out why:

In file included from src/types-compat.h:4:0,
                 from src/math_private.h:23,
                 from src/common.c:3:
/usr/local/Cellar/x64-elf-gcc/4.9.3/lib/gcc/x86_64-elf/4.9.3/include/stdint.h:9:26: fatal error: stdint.h: No such file or directory
 # include_next <stdint.h>
compilation terminated.
make[1]: *** [src/common.c.o] Error 1
make: *** [libstd/openlibm/libopenlibm.a] Error 2