Remove nitpicking mode (-n) for sphinx-build.

sphinx-build v0.6.6 which is on RHEL6/CentOS6 does not know about -n,
so we'll have to do without it for now.

Reported in:	#1535

doc/sphinx/Makefile.am

@@ -3,7 +3,7 @@
 
 # You can set these variables from the command line.
 SPHINXOPTS    =
-SPHINXBUILD   = sphinx-build -W -N -n
+SPHINXBUILD   = sphinx-build -W -N
 PAPER         = a4
 BUILDDIR      = build