Makefile tweaks: Solaris 9+ dont need iconv / move up uname variables

Makefile tweaks: Solaris 9+ dont need iconv / move up uname variables

- Solaris 9 and up do not need -liconv, so NEEDS_LIBICONV should be set
   only for S8.
- Move the declaration of the uname variables to early in the Makefile
   so they can be referenced by prefix and gitexecdir variables.
- gitexecdir defaults to being same as bindir, it might as well reference
   that variable.

[jc: corrupt patch, sneakily tried to remove inclusion of GIT-VERSION-FILE
 I do not know why I am applying this...]

Signed-off-by: Paul Jakma <paul@quagga.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>