Document $MPLSETUPCFG.

Author: anntzer

doc/faq/environment_variables_faq.rst

@@ -35,6 +35,13 @@ Environment variables
   used to find a base directory in which the :file:`matplotlib` subdirectory is
   created.
 
+.. envvar:: MPLSETUPCFG
+
+   This optional variable can be set to the full path of a :file:`setup.cfg`
+   configuration file used to customize the Matplotlib build.  By default, a
+   :file:`setup.cfg` file in the root of the Matplotlib source tree will be
+   read.  Supported build options are listed in :file:`setup.cfg.template`.
+
 .. envvar:: PATH
 
   The list of directories searched to find executable programs.