doc: fallback to sphinx-1.0-build

doc: fallback to sphinx-1.0-build

Some platforms (actually just Centos6, again, of course) call
"sphinx-build" as "sphinx-1.0-build", and so to work around this
apparently useless name, fall back to sphinx-1.0-build when we can't
find sphinx-build before displaying an error message. I am not doing in
this the configure script because the sphinx makefiles are not Automake
files and the less Automake in tree the better.

Signed-off-by: Quentin Young <qlyoung@qlyoung.net>