CMP0022: Warn about a given target at most once

Since cmTarget::ComputeLinkInterface is called separately for each
"head" target that links a target, the warning we produce when
CMP0022 is not set could be repeated.  Add explicit logic to allow
the warning to appear at most once.  Multiple copies of the warning
for the same target are almost always identical and therefore
redundant.  In the rare case that two copies of the warning are
different, the second can appear in a future run after the first
is fixed.

Source/cmTarget.cxx

@@ -83,10 +83,12 @@ class cmTargetInternals
 public:
   cmTargetInternals()
     {
+    this->PolicyWarnedCMP0022 = false;
     this->SourceFileFlagsConstructed = false;
     }
   cmTargetInternals(cmTargetInternals const& r)
     {
+    this->PolicyWarnedCMP0022 = false;
     this->SourceFileFlagsConstructed = false;
     // Only some of these entries are part of the object state.
     // Others not copied here are result caches.
@@ -109,6 +111,7 @@ public:
   typedef std::map<TargetConfigPair, OptionalLinkInterface>
                                                           LinkInterfaceMapType;
   LinkInterfaceMapType LinkInterfaceMap;
+  bool PolicyWarnedCMP0022;
 
   typedef std::map<cmStdString, cmTarget::OutputInfo> OutputInfoMapType;
   OutputInfoMapType OutputInfoMap;
@@ -6433,7 +6436,8 @@ bool cmTarget::ComputeLinkInterface(const char* config, LinkInterface& iface,
       }
     }
 
-  if(explicitLibraries && this->PolicyStatusCMP0022 == cmPolicies::WARN)
+  if(explicitLibraries && this->PolicyStatusCMP0022 == cmPolicies::WARN &&
+     !this->Internal->PolicyWarnedCMP0022)
     {
     // Compare the explicitly set old link interface properties to the
     // preferred new link interface property one and warn if different.
@@ -6455,6 +6459,7 @@ bool cmTarget::ComputeLinkInterface(const char* config, LinkInterface& iface,
         linkIfaceProp << ":\n"
         "  " << (explicitLibraries ? explicitLibraries : "(empty)") << "\n";
       this->Makefile->IssueMessage(cmake::AUTHOR_WARNING, w.str());
+      this->Internal->PolicyWarnedCMP0022 = true;
       }
     }
 
@@ -6544,7 +6549,8 @@ bool cmTarget::ComputeLinkInterface(const char* config, LinkInterface& iface,
       iface.Languages = impl->Languages;
       }
 
-    if(this->PolicyStatusCMP0022 == cmPolicies::WARN)
+    if(this->PolicyStatusCMP0022 == cmPolicies::WARN &&
+       !this->Internal->PolicyWarnedCMP0022)
       {
       // Compare the link implementation fallback link interface to the
       // preferred new link interface property and warn if different.
@@ -6603,6 +6609,7 @@ bool cmTarget::ComputeLinkInterface(const char* config, LinkInterface& iface,
           "Link implementation:\n"
           "  " << oldLibraries << "\n";
         this->Makefile->IssueMessage(cmake::AUTHOR_WARNING, w.str());
+        this->Internal->PolicyWarnedCMP0022 = true;
         }
       }
     }

Tests/RunCMake/CMP0022/CMP0022-NOWARN-exe-stderr.txt

@@ -0,0 +1 @@
+^$

Tests/RunCMake/CMP0022/CMP0022-NOWARN-exe.cmake

@@ -0,0 +1,7 @@
+enable_language(CXX)
+
+add_library(testLib empty_vs6_1.cpp)
+add_executable(testExe empty_vs6_2.cpp)
+target_link_libraries(testExe testLib)
+
+export(TARGETS testExe FILE "${CMAKE_CURRENT_BINARY_DIR}/cmp0022NOWARN-exe.cmake")

Tests/RunCMake/CMP0022/CMP0022-WARN-stderr.txt

@@ -1,4 +1,4 @@
-CMake Warning \(dev\) in CMakeLists.txt:
+^CMake Warning \(dev\) in CMakeLists.txt:
   Policy CMP0022 is not set: INTERFACE_LINK_LIBRARIES defines the link
   interface.  Run "cmake --help-policy CMP0022" for policy details.  Use the
   cmake_policy command to set the policy and suppress this warning.

Tests/RunCMake/CMP0022/CMP0022-WARN.cmake

@@ -9,3 +9,8 @@ set_property(TARGET bar PROPERTY LINK_INTERFACE_LIBRARIES bat)
 
 add_library(user empty.cpp)
 target_link_libraries(user bar)
+
+# Use "bar" again with a different "head" target to check
+# that the warning does not appear again.
+add_library(user2 empty_vs6_3.cpp)
+target_link_libraries(user2 bar)

Tests/RunCMake/CMP0022/RunCMakeTest.cmake

@@ -4,6 +4,7 @@ run_cmake(CMP0022-WARN)
 run_cmake(CMP0022-WARN-tll)
 run_cmake(CMP0022-WARN-static)
 run_cmake(CMP0022-WARN-empty-old)
+run_cmake(CMP0022-NOWARN-exe)
 run_cmake(CMP0022-NOWARN-shared)
 run_cmake(CMP0022-NOWARN-static)
 run_cmake(CMP0022-NOWARN-static-link_libraries)

Tests/RunCMake/CMP0022/empty_vs6_4.cpp

@@ -0,0 +1 @@
+#include "empty.cpp"