mingw32: enable sysrooted toolchains

When targeting mingw32, gcc expects to find its include files
in "mingw/include" instead of the traditional "usr/include".

[Yann E. MORIN: split the original patch]

 # Compute s390-specific values

 CT_DoArchTupleValues() {

     # That's the only thing to override

     if [ "${CT_ARCH_64}" = "y" ]; then

         CT_TARGET_ARCH="s390x"

     fi

 }