Well... it could be done better, but this works for now assuming it's in
/usr/include (and it's correct for mxe builts). Does need proper
autoconfiscation, though.
I got MXE to build (took only an envvar and a couple packages, yay doc
reading), so I thought it time to update the scripts to use it (they assume
/opt/mxe).
CC_FOR_BUILD is the recommended name these days (HOST_CC was from an old
gcc version, and is confusing anyway). Also, CC_FOR_BUILD should be set by
configure.