Document <Project>_TRACE_DEPENDENCY_HANDLING_ONLY (TriBITSPub#597)

This is a variable that project developers and users should know about.

tribits/doc/build_ref/TribitsBuildReferenceBody.rst

@@ -376,6 +376,7 @@ See the following use cases:
 * `Enable all packages (and optionally all tests)`_
 * `Disable a package and all its dependencies`_
 * `Remove all package enables in the cache`_
+* `Speed up debugging dependency handling`_
 
 
 Determine the list of packages that can be enabled
@@ -656,6 +657,22 @@ For example, one would want to do this to avoid more expensive compiler and
 TPL checks.
 
 
+Speed up debugging dependency handling
++++++++++++++++++++++++++++++++++++++++
+
+To speed up debugging the package enable/disable dependency handling, set the
+cache variable::
+
+  -D <Project>_TRACE_DEPENDENCY_HANDLING_ONLY=ON
+
+This will result in only performing the package enable/disable dependency
+handling logic and tracing what would be done to configure the compilers and
+configure the various enabled packages but not actually do that work.  This
+can greatly speed up the time to complete the ``cmake`` configure command when
+debugging the dependency handling (or when creating tests that check that
+behavior).
+
+
 Selecting compiler and linker options
 -------------------------------------