Fix building on macOS/Apple Silicon

[jstasiak]

The build error I got without this patch:

% make
mkdir -p build
gcc -c src/vocab_count.c -o build/vocab_count.o -lm -pthread -O3 -march=native -funroll-loops -Wall -Wextra -Wpedantic
clang: warning: -lm: 'linker' input unused [-Wunused-command-line-argument]
clang: error: the clang compiler does not support '-march=native'
make: *** [build/vocab_count.o] Error 1

[AngledLuffa]

Thanks!