add a defvar for the cmake command

[brianx7]

The environment that I work in uses a wrapper python script to launch cmake (to setup the compiler environment first before running cmake). It would be nice to be able to set a variable, cmake-ide-cmake-command, or something like that (but defaults to cmake), so that there is some customization as to how cmake is run.

[atilaneves]

Intersting use case. I'll add the variable.