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, 4 insertions, 0 deletions

diff --git a/Source/cmCoreTryCompile.cxx b/Source/cmCoreTryCompile.cxx
index ed485e3..1ae7035 100644
--- a/Source/cmCoreTryCompile.cxx
+++ b/Source/cmCoreTryCompile.cxx
@@ -281,6 +281,10 @@ int cmCoreTryCompile::TryCompileCode(std::vector<std::string> const& argv)
       flag += this->Makefile->GetSafeDefinition("CMAKE_OSX_DEPLOYMENT_TARGET");
       cmakeFlags.push_back(flag);
       }
+    if(this->Makefile->GetDefinition("CMAKE_POSITION_INDEPENDENT_CODE")!=0)
+      {
+      fprintf(fout, "SET(CMAKE_POSITION_INDEPENDENT_CODE \"ON\")\n");
+      }
 
     /* Use a random file name to avoid rapid creation and deletion
        of the same executable name (some filesystems fail on that).  */