try_run: Use CMAKE_CROSSCOMPILING_EMULATOR.

If the CMAKE_CROSSCOMPILING_EMULATOR variable is defined, and
CMAKE_CROSSCOMPILING is TRUE, then use CMAKE_CROSSCOMPILING_EMULATOR to run
the try_run executables. This prevents the need to populate
TryRunResults.cmake when cross compiling.

Help/command/try_run.rst

@@ -73,7 +73,8 @@ When cross compiling, the executable compiled in the first step
 usually cannot be run on the build host.  The ``try_run`` command checks
 the :variable:`CMAKE_CROSSCOMPILING` variable to detect whether CMake is in
 cross-compiling mode.  If that is the case, it will still try to compile
-the executable, but it will not try to run the executable.  Instead it
+the executable, but it will not try to run the executable unless the
+:variable:`CMAKE_CROSSCOMPILING_EMULATOR` variable is set.  Instead it
 will create cache variables which must be filled by the user or by
 presetting them in some CMake script file to the values the executable
 would have produced if it had been run on its actual target platform.

Help/variable/CMAKE_CROSSCOMPILING_EMULATOR.rst

@@ -1,5 +1,12 @@
 CMAKE_CROSSCOMPILING_EMULATOR
 -----------------------------
 
-Default value for the :prop_tgt:`CROSSCOMPILING_EMULATOR` target property of
-executables.  See that target property for additional information.
+This variable is only used when :variable:`CMAKE_CROSSCOMPILING` is on. It
+should point to a command on the host system that can run executable built
+for the target system.
+
+The command will be used to run :command:`try_run` generated executables,
+which avoids manual population of the TryRunResults.cmake file.
+
+It is also used as the default value for the
+:prop_tgt:`CROSSCOMPILING_EMULATOR` target property of executables.