Refactor generation of shared library flags

CMAKE_SHARED_LIBRARY_<lang>_FLAGS has flags on various platforms for a
variety of purposes that are correlated with shared libraries but not
exclusive to them.  Refactor generation of these flags to use new
purpose-specific platform variables

  CMAKE_<lang>_COMPILE_OPTIONS_DLL
  CMAKE_<lang>_COMPILE_OPTIONS_PIC
  CMAKE_<lang>_COMPILE_OPTIONS_PIE

Activate the DLL flags specifically for shared libraries.  Add a new
POSITION_INDEPENDENT_CODE target property to activate PIC/PIE flags, and
default to true for shared libraries to preserve default behavior.
Initialize the new property from CMAKE_POSITION_INDEPENDENT_CODE to
allow easy global configuration in projects.

Although the default behavior is unchanged by this refactoring, the new
approach ignores CMAKE_SHARED_LIBRARY_<lang>_FLAGS completely.  We must
leave it set in case projects reference the value.  Furthermore, if a
project modifies CMAKE_SHARED_LIBRARY_<lang>_FLAGS it expects the new
value to be used.  Add policy CMP0018 to handle compatibility with
projects that modify this platform variable.

Add a PositionIndependentCode test on platforms where we can get
meaningful results.

1 files changed, 21 insertions, 0 deletions

diff --git a/Source/cmGlobalGenerator.cxx b/Source/cmGlobalGenerator.cxx
index b06cdb4..f3a7090 100644
--- a/Source/cmGlobalGenerator.cxx
+++ b/Source/cmGlobalGenerator.cxx
@@ -586,6 +586,16 @@ cmGlobalGenerator::EnableLanguage(std::vector<std::string>const& languages,
           }
         } // end if in try compile
       } // end need test language
+    // Store the shared library flags so that we can satisfy CMP0018
+    std::string sharedLibFlagsVar = "CMAKE_SHARED_LIBRARY_";
+    sharedLibFlagsVar += lang;
+    sharedLibFlagsVar += "_FLAGS";
+    const char* sharedLibFlags =
+      mf->GetSafeDefinition(sharedLibFlagsVar.c_str());
+    if (sharedLibFlags)
+      {
+      this->LanguageToOriginalSharedLibFlags[lang] = sharedLibFlags;
+      }
     } // end for each language
 
   // Now load files that can override any settings on the platform or for
@@ -2107,6 +2117,17 @@ cmGlobalGenerator::GenerateRuleFile(std::string const& output) const
 }
 
 //----------------------------------------------------------------------------
+std::string cmGlobalGenerator::GetSharedLibFlagsForLanguage(
+                                                        std::string const& l)
+{
+  if(this->LanguageToOriginalSharedLibFlags.count(l) > 0)
+    {
+    return this->LanguageToOriginalSharedLibFlags[l];
+    }
+  return "";
+}
+
+//----------------------------------------------------------------------------
 void cmGlobalGenerator::AppendDirectoryForConfig(const char*, const char*,
                                                  const char*, std::string&)
 {