From MSDN:
cl.exe is a tool that controls the Microsoft C and C++ compilers and linker. cl.exe can be run only on operating systems that support Microsoft Visual Studio.
You should run luarocks install
inside the Visual Studio Command Prompt.
I tried to install moonscript without Visual Studio and found that moonscript does not have a binary distribution.
Anyway, I found the binary package for each of moonscript's dependencies, the installation procedure is the follwing:
luarocks install lpeg 0.10-1 # this is the one that failed first
luarocks install luafilesystem 1.6.0-1 # this one failed after installing lpeg
But the following:
luarocks install moonscript
Can only be done within the Visual Studio Command Prompt unless you manually download a binary package.