Document CMAKE_TRY_COMPILE_CONFIGURATION variable

Also reference it from try_compile and try_run since it affects those
commands.

Source/cmDocumentVariables.cxx

@@ -1121,6 +1121,14 @@ void cmDocumentVariables::DefineVariables(cmake* cm)
      "this variable for a target if they are set.  "
      "Library targets are otherwise placed in this directory.",false,
      "Variables that Control the Build");
+  cm->DefineProperty
+    ("CMAKE_TRY_COMPILE_CONFIGURATION", cmProperty::VARIABLE,
+     "Build configuration used for try_compile and try_run projects.",
+     "Projects built by try_compile and try_run are built "
+     "synchronously during the CMake configuration step.  "
+     "Therefore a specific build configuration must be chosen even "
+     "if the generated build system supports multiple configurations.",false,
+     "Variables that Control the Build");
 
 
 //   Variables defined when the a language is enabled These variables will

Source/cmTryCompileCommand.h

@@ -94,7 +94,9 @@ public:
       "Return the success or failure in "
       "RESULT_VAR. CMAKE_FLAGS can be used to pass -DVAR:TYPE=VALUE flags "
       "to the cmake that is run during the build. "
-      "";
+      "Set variable CMAKE_TRY_COMPILE_CONFIGURATION to choose a build "
+      "configuration."
+      ;
     }
   
   cmTypeMacro(cmTryCompileCommand, cmCoreTryCompile);

Source/cmTryRunCommand.h

@@ -93,7 +93,10 @@ public:
       "that when crosscompiling, the cache variables will have to be set "
       "manually to the output of the executable. You can also \"guard\" the "
       "calls to try_run with if(CMAKE_CROSSCOMPILING) and provide an "
-      "easy-to-preset alternative for this case.\n";
+      "easy-to-preset alternative for this case.\n"
+      "Set variable CMAKE_TRY_COMPILE_CONFIGURATION to choose a build "
+      "configuration."
+      ;
     }
 
   cmTypeMacro(cmTryRunCommand, cmCoreTryCompile);