configure: bootstrap sphinx with mkvenv

When docs are explicitly requested, require Sphinx>=1.6.0. When docs are
explicitly disabled, don't bother to check for Sphinx at all. If docs
are set to "auto", attempt to locate Sphinx, but continue onward if it
wasn't located.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20230511035435.734312-22-jsnow@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

configure

@@ -1115,9 +1115,9 @@ fi
 
 # Suppress writing compiled files
 python="$python -B"
+mkvenv="$python ${source_path}/python/scripts/mkvenv.py"
 
-
-if ! $python "${source_path}/python/scripts/mkvenv.py" ensure \
+if ! $mkvenv ensure \
      --dir "${source_path}/python/wheels" \
      --diagnose "meson" \
      "meson>=0.61.5" ;
@@ -1132,6 +1132,23 @@ fi
 
 meson="$(cd pyvenv/bin; pwd)/meson"
 
+# Conditionally ensure Sphinx is installed.
+
+if test "$docs" != "disabled" ; then
+    if ! $mkvenv ensure \
+         --diagnose "sphinx-build" \
+         "sphinx>=1.6.0" "sphinx-rtd-theme>=0.5.0";
+    then
+        if test "$docs" = "enabled" ; then
+            exit 1
+        fi
+        echo "Sphinx not found/usable, disabling docs."
+        docs=disabled
+    else
+        docs=enabled
+    fi
+fi
+
 # Probe for ninja
 
 if test -z "$ninja"; then