The preprocessor directive BUILD_TclOO doesn't end up in config.h, which breaks cross-building for windows platforms from linux using a cross gcc.
This patch provides a manually adapted configure.in, and a new config.in regenerated from configure.in (by calling autoheader). The redundancy[*] that this patch introduces in the processing done by the resulting ./configure script is taken for granted.
The patch was produced against the head of the trunk at 17-jun-2017 (sha1: e5b56214c7d658e95d2af367ab82d7a96b5a9afb).
[*] ./configure script will now add the BUILD_TclOO directive twice to an intermediate file confdefs.h which is used to produce config.h. So be it, most users won't ever notice.