cmake_host_system_information: Also set USED_FALLBACK_SCRIPT

If the OS identification was obtained via fallback script.

Source/cmCMakeHostSystemInformationCommand.cxx

@@ -287,6 +287,8 @@ std::map<std::string, std::string> GetOSReleaseVariables(
 
   // Ugh, it could be some pre-os-release distro.
   // Lets try some fallback getters.
+  // See also:
+  //  - http://linuxmafia.com/faq/Admin/release-files.html
 
   // 1. CMake provided
   cmsys::Glob gl;
@@ -358,8 +360,9 @@ std::map<std::string, std::string> GetOSReleaseVariables(
       data.emplace(std::move(key), std::move(value));
     }
 
+    // Try 'till some script can get anything
     if (!data.empty()) {
-      // Try 'till some script can get anything
+      data.emplace("USED_FALLBACK_SCRIPT", script);
       break;
     }
   }

Tests/RunCMake/cmake_host_system_information/CentOS6-stdout.txt

@@ -2,5 +2,6 @@
 -- CENTOS6_ID_LIKE=`rhel`
 -- CENTOS6_NAME=`CentOS`
 -- CENTOS6_PRETTY_NAME=`CentOS release 6\.10 \(Final\)`
+-- CENTOS6_USED_FALLBACK_SCRIPT=`.*/Modules/Internal/OSRelease/010-TryOldCentOS.cmake`
 -- CENTOS6_VERSION=`6\.10`
 -- CENTOS6_VERSION_ID=`6\.10`

Tests/RunCMake/cmake_host_system_information/Debian6-stdout.txt

@@ -1,4 +1,5 @@
 -- DEBIAN6_ID=`debian`
 -- DEBIAN6_NAME=`Debian`
+-- DEBIAN6_USED_FALLBACK_SCRIPT=`.*/Modules/Internal/OSRelease/020-TryDebianVersion.cmake`
 -- DEBIAN6_VERSION=`6\.0\.10`
 -- DEBIAN6_VERSION_ID=`6\.0\.10`

Tests/RunCMake/cmake_host_system_information/UserFallbackScript-stdout.txt

@@ -2,5 +2,6 @@
 -- UFS_ID_LIKE=`nothing`
 -- UFS_NAME=`UnitTest`
 -- UFS_PRETTY_NAME=`Just a Unit Test`
+-- UFS_USED_FALLBACK_SCRIPT=`.*/999-LastFallbackScript\.cmake`
 -- UFS_VERSION=`0\.0\.1`
 -- UFS_VERSION_ID=`0\.0\.1`