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, 15 insertions, 0 deletions

diff --git a/Source/cmTarget.cxx b/Source/cmTarget.cxx
index c5ef9ff..4f3f2c5 100644
--- a/Source/cmTarget.cxx
+++ b/Source/cmTarget.cxx
@@ -756,6 +756,14 @@ void cmTarget::DefineProperties(cmake *cm)
      "(such as \"lib\") on a library name.");
 
   cm->DefineProperty
+    ("POSITION_INDEPENDENT_CODE", cmProperty::TARGET,
+     "Whether to create a position-independent target",
+     "The POSITION_INDEPENDENT_CODE property determines whether position "
+     "independent executables or shared libraries will be created.  "
+     "This property is true by default for SHARED and MODULE library "
+     "targets and false otherwise.");
+
+  cm->DefineProperty
     ("POST_INSTALL_SCRIPT", cmProperty::TARGET,
      "Deprecated install support.",
      "The PRE_INSTALL_SCRIPT and POST_INSTALL_SCRIPT properties are the "
@@ -1305,6 +1313,13 @@ void cmTarget::SetMakefile(cmMakefile* mf)
   this->SetProperty("INCLUDE_DIRECTORIES",
                     this->Makefile->GetProperty("INCLUDE_DIRECTORIES"));
 
+  if(this->TargetTypeValue == cmTarget::SHARED_LIBRARY
+      || this->TargetTypeValue == cmTarget::MODULE_LIBRARY)
+    {
+    this->SetProperty("POSITION_INDEPENDENT_CODE", "True");
+    }
+  this->SetPropertyDefault("POSITION_INDEPENDENT_CODE", 0);
+
   // Record current policies for later use.
   this->PolicyStatusCMP0003 =
     this->Makefile->GetPolicyStatus(cmPolicies::CMP0003);