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, 3 insertions, 0 deletions

diff --git a/Source/cmGlobalGenerator.h b/Source/cmGlobalGenerator.h
index 5254b89..0ac38a0 100644
--- a/Source/cmGlobalGenerator.h
+++ b/Source/cmGlobalGenerator.h
@@ -277,6 +277,8 @@ public:
       i.e. "Can I build Debug and Release in the same tree?" */
   virtual bool IsMultiConfig() { return false; }
 
+  std::string GetSharedLibFlagsForLanguage(std::string const& lang);
+
   /** Generate an <output>.rule file path for a given command output.  */
   virtual std::string GenerateRuleFile(std::string const& output) const;
 
@@ -357,6 +359,7 @@ private:
   std::map<cmStdString, cmStdString> LanguageToOutputExtension;
   std::map<cmStdString, cmStdString> ExtensionToLanguage;
   std::map<cmStdString, int> LanguageToLinkerPreference;
+  std::map<cmStdString, cmStdString> LanguageToOriginalSharedLibFlags;
 
   // Record hashes for rules and outputs.
   struct RuleHash { char Data[32]; };