Fix documentation building rules

Implement the correct behavior to be kept when sphinx-build is not installed.
It was already correctly checked, but instead of just disabling the
documentation generation, an error message was printed, having "make" fail.
Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Thomas Thrainer <thomasth@google.com>

Makefile.am

@@ -745,14 +745,17 @@ endif
 # Things to build but not to install (add it to EXTRA_DIST if it should be
 # distributed)
 noinst_DATA = \
-	doc/html \
 	$(BUILT_EXAMPLES) \
 	doc/examples/bash_completion \
 	doc/examples/bash_completion-debug \
 	$(manhtml)
 
+if HAS_SPHINX
 if MANPAGES_IN_DOC
 noinst_DATA += doc/man-html
+else
+noinst_DATA += doc/html
+endif
 endif
 
 gnt_scripts = \

configure.ac

@@ -408,6 +408,7 @@ else
     AC_MSG_ERROR([Sphinx 1.0 or higher is required])
   fi
 fi
+AM_CONDITIONAL([HAS_SPHINX], [test -n "$SPHINX"])
 
 AC_ARG_ENABLE([manpages-in-doc],
   [AS_HELP_STRING([--enable-manpages-in-doc],