Replace call to mawk by awk.

mawk handling of `\.` is different from gawk:
gawk complains about `warning: escape sequence '\.' treated as plain '.'`, and does not create the right pattern.

New version works identically with both, but we need to fight against the 2 double-quotes to get the backslash to what we want...

Rationale behind this change is to be indepedent of the awk flavour whenever we can, as Arch only ships `gawk` (this is easy, `croutonwheel` will be harder).