building windows packages under wine/linux and cross-compiling.

Had some fun today, and thought it might be a good idea to share
and possibly for inclusion to R/src/gnuwin32/README.packages .

[...]

Cross-compiling: The instruction in R/src/gnuwin32/README.packages 
essentially works, with one missing detail: "R_EXE=/usr/bin/R" is also 
needed. Thus it should be:

     make R_EXE=/usr/bin/R PKGDIR=/mysources RLIB=/R/win/library \
             pkg-mypkg
     make P_EXE=/usr/bin/R KGDIR=/mysources RLIB=/R/win/library \
             pkgcheck-mypkg

The instructions do work for those who actually follow them! That file 
says

  Edit MkRules to set BUILD=CROSS and the appropriate paths (including
  HEADER) as needed.

and the appropriate section of that file is

## =================== cross-compilation settings  =================

ifeq ($(strip $(BUILD)),CROSS)
# Next might be mingw32- or i386-mingw32msvc- or i586-
# depending on the cross-compiler.
BINPREF=i586-mingw32-
# Set this to where the mingw32 include files are. It must be accurate.
HEADER=/users/ripley/R/cross-tools5/i586-mingw32/include
endif
# path (possibly full path) to same version of R on the host system
# R_EXE=R

and please do note the last two lines.

Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595