Getting the value of a user-defined configure variable into config.h

[Brian]

For debugging purposes, I would like to have the value of the current
build's svn revision in the source code. I have written a macro to do
this, however, I cannot get the value of $(SVN_REV) into config.h. I
was previously able to get $(datadir) into my makefiles through the
following method:

CXXFLAGS="$CXXFLAGS -DWHEREAMI=\\\"\$(datadir)\\\""

However, I have found no equivalent for my current problem. I have
tried to mimic the code for AC_DEFINE_DIR, but I am not sure what the
following lines do (actually, the entire macro is somewhat of a
mystery!):

eval ac_define_dir="\"[$]$2\""
eval ac_define_dir="\"$ac_define_dir\""

Thanks,
Brian