[sdks] Debian Linux doesn't need to build MXE
Debian and its derivatives include full mingw chain (both 64 and 32-bit) and so
they don't need to build the toolchain at all. However, the current detection of
Linux flavor takes into account only Ubuntu and all other distros are configured
to *require* MXE but they do *not* build them.
This commit fixes detection of distros which provide the mingw toolchain in a
way that makes it easy to extend the set of compatible distributions as needed.
In order to mark a distribution as one with MinGW one has to add the `:Distro:`
string to the `LINUX_WITH_MINGW` make variable in `mxe.mk`. The `Distro` string
is whatever the `determine-linux-flavor.sh` script returns.