- change the interpretation of FPTOOLS_TOP_ABS, it is now the UNIXy path.
  For Win32, it is of the form <drive>:<path> where '/' is the directory
  separator. Prior to this commit, the directory separator was '\'.
- for the (only) place we need to have a platform-native representation
  of FPTOOLS_TOP_ABS, use FPTOOLS_TOP_ABS_PLATFORM.