Check whether a configuration is currently active.
Details
The name of the currently active configuration is read from the
R_CONFIG_ACTIVE environment variable. If the variable is not defined then
the "default" configuration is used.
To test for whether a configuration is active you should use the
is_active() function rather than inspecting the environment variable
directly (this is to so that tests remain valid if other means of specifying
configurations are introduced in the future).