c-label-minimum-indentation is a variable defined in `cc-vars.el'.
Its value is

*Minimum indentation for lines inside code blocks.
This variable typically only affects code using the `gnu' style, which
mandates a minimum of one space in front of every line inside code
blocks. Specifically, the function `c-gnu-impose-minimum' on your
`c-special-indent-hook' is what enforces this.

This is a style variable. Apart from the valid values described
above, it can be set to the symbol `set-from-style'. In that case,
it takes its value from the style system (see `c-default-style' and
`c-style-alist') when a CC Mode buffer is initialized. Otherwise,
the value set here overrides the style system (there is a variable
`c-old-style-variable-behavior' that changes this, though).

You can customize this variable.