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, 28 insertions, 0 deletions

diff --git a/Source/cmPolicies.cxx b/Source/cmPolicies.cxx
index 37070b6..79af4d7 100644
--- a/Source/cmPolicies.cxx
+++ b/Source/cmPolicies.cxx
@@ -463,6 +463,34 @@ cmPolicies::cmPolicies()
     "The OLD behaviour is to always prefer files from CMAKE_MODULE_PATH over "
     "files from the CMake modules directory.",
     2,8,4,0, cmPolicies::WARN);
+
+    this->DefinePolicy(
+    CMP0018, "CMP0018",
+    "Ignore CMAKE_SHARED_LIBRARY_<Lang>_FLAGS variable.",
+    "CMake 2.8.8 and lower compiled sources in SHARED and MODULE libraries "
+    "using the value of the undocumented CMAKE_SHARED_LIBRARY_<Lang>_FLAGS "
+    "platform variable.  The variable contained platform-specific flags "
+    "needed to compile objects for shared libraries.  Typically it included "
+    "a flag such as -fPIC for position independent code but also included "
+    "other flags needed on certain platforms.  CMake 2.8.9 and higher "
+    "prefer instead to use the POSITION_INDEPENDENT_CODE target property to "
+    "determine what targets should be position independent, and new "
+    "undocumented platform variables to select flags while ignoring "
+    "CMAKE_SHARED_LIBRARY_<Lang>_FLAGS completely."
+    "\n"
+    "The default for either approach produces identical compilation flags, "
+    "but if a project modifies CMAKE_SHARED_LIBRARY_<Lang>_FLAGS from its "
+    "original value this policy determines which approach to use."
+    "\n"
+    "The OLD behavior for this policy is to ignore the "
+    "POSITION_INDEPENDENT_CODE property for all targets and use the modified "
+    "value of CMAKE_SHARED_LIBRARY_<Lang>_FLAGS for SHARED and MODULE "
+    "libraries."
+    "\n"
+    "The NEW behavior for this policy is to ignore "
+    "CMAKE_SHARED_LIBRARY_<Lang>_FLAGS whether it is modified or not and "
+    "honor the POSITION_INDEPENDENT_CODE target property.",
+    2,8,9,0, cmPolicies::WARN);
 }
 
 cmPolicies::~cmPolicies()