Explained what CMake options from OST are to be considered

doc/buildsystem.rst

@@ -46,6 +46,15 @@ Here is a list of more options used within |project|:
 * ``DISABLE_LINKCHECK`` Don't test links from documentation on ``make check``
   (implicit by ``DISABLE_DOCUMENTATION``)
 
+Since we use |ost_s| in the background, some of its options for |cmake| are
+also relevant, here. Basically they need to be set to exactly the same value.
+Even if |project| builds with different settings, it may start producing funny
+results at an unexpected point. If you do not know the values, grep the option
+in the :file:`CMakeCache.txt` of |ost_s|:
+
+* ``OPTIMIZE``
+* ``OST_DOUBLE_PRECISION``
+
 Instead of calling |cmake| by yourself, there is the :file:`conf-scripts`
 directory, providing smallish scripts to invoke |cmake| the right way for
 various systems. Usually those scripts just need the |ost_s| and |qmean| paths
@@ -97,4 +106,4 @@ special targets:
 .. _eigen3: http://eigen.tuxfamily.org/
 
 ..  LocalWords:  cmake makefiles CMakeLists txt DOCTEST LINKCHECK conf html
-..  LocalWords:  doctest linkcheck qmean DQMEAN eigen
+..  LocalWords:  doctest linkcheck qmean DQMEAN eigen CMakeCache