Set minimum gettext version to 0.17.

Set minimum gettext version to 0.17.

This fixes the problem that the 1.0.69 tarball would fail in
the po/ subdirectory when installing:

make[1]: Entering directory `/builddir/build/BUILD/libguestfs-1.0.69/po'
/bin/sh @MKINSTALLDIRS@ /builddir/build/BUILDROOT/libguestfs-1.0.69-1.fc11.x86_64/usr/share
/bin/sh: @MKINSTALLDIRS@: No such file or directory
make[1]: Leaving directory `/builddir/build/BUILD/libguestfs-1.0.69/po'

RHEL 5.3 has gettext 0.14.6.  However the tarball will still
build (since it includes all the required code).  This may cause
a problem if RHEL users want to build from the git repository.