Why dependencies yaml? Mac luajit build error.

Hi @jgm

I wanted to use lcmark and was looking forward to the luajit support.

But, depency, yaml has build error.

Although yaml has not been maintained for a long time, but
Is there a problem with using a maintained library such as lyaml (https://github.com/gvvaughan/lyaml)?

```
$ luarocks install lcmark
Warning: falling back to wget - install luasec to get native HTTPS support
Installing https://luarocks.org/lcmark-0.29.0-5.src.rock
Missing dependencies for lcmark 0.29.0-5:
yaml >= 1.1 (not installed)
optparse >= 1.0.1 (not installed)

lcmark 0.29.0-5 depends on lua >= 5.1 (5.1-1 provided by VM)
lcmark 0.29.0-5 depends on cmark >= 0.29.0 (0.29.0-1 installed)
lcmark 0.29.0-5 depends on yaml >= 1.1 (not installed)
Installing https://luarocks.org/yaml-1.1.2-1.src.rock

yaml 1.1.2-1 depends on lua >= 5.1, < 5.4 (5.1-1 provided by VM)
yaml 1.1.2-1 depends on lub >= 1.0.3, < 2 (1.1.0-1 installed)
env MACOSX_DEPLOYMENT_TARGET=10.8 gcc -O2 -fPIC -I/usr/local/include/luajit-2.1 -c src/api.c -o src/api.o -Isrc
env MACOSX_DEPLOYMENT_TARGET=10.8 gcc -O2 -fPIC -I/usr/local/include/luajit-2.1 -c src/b64.c -o src/b64.o -Isrc
env MACOSX_DEPLOYMENT_TARGET=10.8 gcc -O2 -fPIC -I/usr/local/include/luajit-2.1 -c src/dumper.c -o src/dumper.o -Isrc
env MACOSX_DEPLOYMENT_TARGET=10.8 gcc -O2 -fPIC -I/usr/local/include/luajit-2.1 -c src/emitter.c -o src/emitter.o -Isrc
env MACOSX_DEPLOYMENT_TARGET=10.8 gcc -O2 -fPIC -I/usr/local/include/luajit-2.1 -c src/loader.c -o src/loader.o -Isrc
env MACOSX_DEPLOYMENT_TARGET=10.8 gcc -O2 -fPIC -I/usr/local/include/luajit-2.1 -c src/lyaml.c -o src/lyaml.o -Isrc
src/lyaml.c:593:15: error: implicit declaration of function 'luaL_getn' is
invalid in C99 [-Werror,-Wimplicit-function-declaration]
int i, n = luaL_getn(dumper->L, -1);
^
src/lyaml.c:660:11: error: implicit declaration of function 'luaL_getn' is
invalid in C99 [-Werror,-Wimplicit-function-declaration]
luaL_getn(dumper->L, -1) > 0) {
^
2 errors generated.

Error: Failed installing dependency: https://luarocks.org/yaml-1.1.2-1.src.rock - Build error: Failed compiling object src/lyaml.o
```