doc: document variable SOCL_OCL_LIB_OPENCL

doc/chapters/configuration.texi

@@ -252,6 +252,7 @@ Disable the build of examples.
 @menu
 * Workers::                     Configuring workers
 * Scheduling::                  Configuring the Scheduling engine
+* Extensions::
 * Misc::                        Miscellaneous and debug
 @end menu
 
@@ -409,6 +410,18 @@ the coefficient to be applied to it before adding it to the computation part.
 
 @end table
 
+@node Extensions
+@subsection Extensions
+
+@table @code
+
+@item @code{SOCL_OCL_LIB_OPENCL}
+THE SOCL test suite is only run when the environment variable
+@code{SOCL_OCL_LIB_OPENCL} is defined. It should contain the location
+of the libOpenCL.so file of the OCL ICD implementation.
+
+@end table
+
 @node Misc
 @subsection Miscellaneous and debug