docs/conf.py: Don't hard-code QEMU version

Don't hard-code the QEMU version number into conf.py. Instead
we either pass it to sphinx-build on the command line, or
(if doing a standalone Sphinx run in a readthedocs.org setup)
extract it from the VERSION file.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190305172139.32662-12-peter.maydell@linaro.org
Message-id: 20190228145624.24885-12-peter.maydell@linaro.org
diff --git a/docs/conf.py b/docs/conf.py
index f452e42..befbcc6 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -76,11 +76,22 @@
 # The version info for the project you're documenting, acts as replacement for
 # |version| and |release|, also used in various other places throughout the
 # built documents.
-#
-# The short X.Y version.
-version = u'4.0'
-# The full version, including alpha/beta/rc tags.
-release = u'4.0'
+
+# Extract this information from the VERSION file, for the benefit of
+# standalone Sphinx runs as used by readthedocs.org. Builds run from
+# the Makefile will pass version and release on the sphinx-build
+# command line, which override this.
+try:
+    extracted_version = None
+    with open(os.path.join(qemu_docdir, '../VERSION')) as f:
+        extracted_version = f.readline().strip()
+except:
+    pass
+finally:
+    if extracted_version:
+        version = release = extracted_version
+    else:
+        version = release = "unknown version"
 
 # The language for content autogenerated by Sphinx. Refer to documentation
 # for a list of supported languages.