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 Alpha-specific values

 CT_DoArchTupleValues () {

     # The architecture part of the tuple:

     CT_TARGET_ARCH="${CT_ARCH}${CT_ARCH_ALPHA_VARIANT}"

 }