Make libiff build on OSX

Currently libiff doesn't build properly on OSX because the version number created in 
the configure script ends with a newline. The error message from configure is:

./configure: line 4: .: filename argument required
.: usage: . filename [arguments]

To fix it I simply used m4_esyscmd_s instead of m4_esyscmd in configure.ac. 
This is probably a good idea anyway, it requires autoconf 2.64 or newer though. 
If you need it to build with older autoconf versions you can get rid of the superfluous 
newline in other ways, as describe in the following discussion:
http://lists.gnu.org/archive/html/autoconf/2010-06/msg00007.html

To build libiff successfully on OSX Mavericks, I did the following:
glibtoolize
autoscan
aclocal
autoheader
autoconf
automake --add-missing
./configure
make

Note the use of glibtoolize instead of libtoolize which took me a while to
figure out. I know this is OSX specific but a hint in the documentation
could save people time.
Maybe not all steps are necessary, but glibtoolize certainly was.

Maybe you could add the configure script to the repo, then the build would work like it is described in
the documentation (README.md).

Thanks for creating libiff.