ENH: Re-enable system include dir suppression

This creates variable CMAKE_<LANG>_IMPLICIT_INCLUDE_DIRECTORIES to
specify implicit include directories on a per-language basis.  This
replaces the previous platform-wide variable.  It is necessary to
avoid explicit specification of -I/usr/include on some compilers
(such as HP aCC) because:

  1.) It may break ordering among system include directories defined
      internally by the compiler, thus getting wrong system headers.
  2.) It tells the compiler to treat the system include directory
      as a user include directory, enabling warnings in the headers.

See issue #8598.

Modules/Platform/UnixPaths.cmake

@@ -53,5 +53,12 @@ LIST(APPEND CMAKE_PLATFORM_IMPLICIT_LINK_DIRECTORIES
   /lib /usr/lib /usr/lib32 /usr/lib64
   )
 
+LIST(APPEND CMAKE_C_IMPLICIT_INCLUDE_DIRECTORIES
+  /usr/include
+  )
+LIST(APPEND CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES
+  /usr/include
+  )
+
 # Enable use of lib64 search path variants by default.
 SET_PROPERTY(GLOBAL PROPERTY FIND_LIBRARY_USE_LIB64_PATHS TRUE)

Source/cmDocumentVariables.cxx

@@ -1097,6 +1097,15 @@ void cmDocumentVariables::DefineVariables(cmake* cm)
      "This is a list of file extensions that may be "
      "part of a project for a given language but are not compiled. ",false,
      "Variables for Languages");
+
+  cm->DefineProperty
+    ("CMAKE_<LANG>_IMPLICIT_INCLUDE_DIRECTORIES", cmProperty::VARIABLE,
+     "Directories implicitly searched by the compiler for header files.",
+     "CMake does not explicitly specify these directories on compiler "
+     "command lines for language <LANG>.  "
+     "This prevents system include directories from being treated as user "
+     "include directories on some compilers.", false,
+     "Variables for Languages");
   
   cm->DefineProperty
     ("CMAKE_<LANG>_LINKER_PREFERENCE", cmProperty::VARIABLE,

Source/cmLocalGenerator.cxx

@@ -1186,6 +1186,23 @@ const char* cmLocalGenerator::GetIncludeFlags(const char* lang)
     {
     return this->LanguageToIncludeFlags[lang].c_str();
     }
+
+  // Load implicit include directories for this language.
+  std::set<cmStdString> impDirs;
+  std::string impDirVar = "CMAKE_";
+  impDirVar += lang;
+  impDirVar += "_IMPLICIT_INCLUDE_DIRECTORIES";
+  if(const char* value = this->Makefile->GetDefinition(impDirVar.c_str()))
+    {
+    std::vector<std::string> impDirVec;
+    cmSystemTools::ExpandListArgument(value, impDirVec);
+    for(std::vector<std::string>::const_iterator i = impDirVec.begin();
+        i != impDirVec.end(); ++i)
+      {
+      impDirs.insert(*i);
+      }
+    }
+
   cmOStringStream includeFlags;
   std::vector<std::string> includes;
   this->GetIncludeDirectories(includes);
@@ -1233,6 +1250,11 @@ const char* cmLocalGenerator::GetIncludeFlags(const char* lang)
 #endif
   for(i = includes.begin(); i != includes.end(); ++i)
     {
+    // Skip implicit include directories.
+    if(impDirs.find(*i) != impDirs.end())
+      {
+      continue;
+      }
 #ifdef __APPLE__
     if(cmSystemTools::IsPathToFramework(i->c_str()))
       {