Source changes as a result of working to port Octave for Android

[Corbin Champion]

All source code modifications necessary, so far, to port Octave to Android have been related to Android not being a "normal" distribution of Linux.

1)  Android doesn't have /tmp.  To this end, could we have get_P_tmpdir() (found within src/sysdep.cc) use the environment variable TMPDIR if present, like what is done by tmpnam (eventually calls, liboctave/tempname.c)?  

2)  Android doesn't have sh found at /bin/sh , instead it is at /system/bin/sh (not my fault).  This is a problem in toplev.cc and in oct-procbuf.cc.  Can this be changed to being dependent on a ifdef based on some variable, such that it defaults to /bin/sh if not already defined?  This way it can be overridden as part of the configure/make flow.  glibc has this same issue in some spots and uses a ifdef in others, so not consistent from what I saw.

FYI...all non-gui required and optional dependencies along with octave itself built and working (more testing necessary to full qualify this) on Android.  

Thanks,

Corbin