-version
- **Default Value = 1**.
- Specifies the ABI version of the library, and is used as the major
- version number of the resulting ``.so`` library.
+reason
+ **Default Value = '<unknown reason>'**.
+ This variable should be used when a library is not to be built i.e. when
+ ``build`` is set to "false", to specify the reason why a library will not be
+ built. For missing dependencies this should be of the form
+ ``'missing dependency, "libname"'``.
+
+use_function_versioning
+ **Default Value = false**.
+ Specifies if the library in question has ABI versioned functions. If it
+ has, this value should be set to ensure that the C files are compiled
+ twice with suitable parameters for each of shared or static library
+ builds.