cmake build system doesnt take variables from LUAROCKS_CONFIG

[teto]

I am trying to fix a build with libluv on nixos. To achieve this, I need to pass certain flags to cmake.
Peeking at src/luarocks/build/cmake.lua, I found that variables are passed to cmake so I proceeded to add variables to the file pointed at by LUAROCKS_CONFIG.
There were ignored and after doublechecking, it seems like src/luarocks/build/cmake.lua takes its variables from the rockspec only, but make has a different behaviors and follows the doc more closely
as variables can also be overriden on the command line src/luarocks/build/make.lua.
Would be cool to have these converge.

[teto]

Related:
#339
#509