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, 37 insertions, 0 deletions

diff --git a/Tests/PositionIndependentTargets/global/CMakeLists.txt b/Tests/PositionIndependentTargets/global/CMakeLists.txt
new file mode 100644
index 0000000..1d662f8
--- /dev/null
+++ b/Tests/PositionIndependentTargets/global/CMakeLists.txt
@@ -0,0 +1,37 @@
+
+set(CMAKE_POSITION_INDEPENDENT_CODE True)
+
+add_executable(test_target_executable_global
+  "${CMAKE_CURRENT_SOURCE_DIR}/../pic_main.cpp"
+)
+
+add_library(test_target_shared_library_global
+  SHARED "${CMAKE_CURRENT_SOURCE_DIR}/../pic_lib.cpp"
+)
+set_target_properties(test_target_shared_library_global
+  PROPERTIES DEFINE_SYMBOL PIC_TEST_BUILD_DLL
+)
+
+add_library(test_target_static_library_global
+  STATIC "${CMAKE_CURRENT_SOURCE_DIR}/../pic_lib.cpp"
+)
+set_target_properties(test_target_static_library_global
+  PROPERTIES COMPILE_DEFINITIONS PIC_TEST_STATIC_BUILD
+)
+
+
+file(READ
+  "${CMAKE_CURRENT_SOURCE_DIR}/../pic_test.h"
+  PIC_HEADER_CONTENT
+)
+
+check_cxx_source_compiles(
+  "
+${PIC_HEADER_CONTENT}
+int main(int,char**) { return 0; }\n"
+  PIC_TRY_COMPILE_RESULT
+)
+
+if (NOT PIC_TRY_COMPILE_RESULT)
+  message(SEND_ERROR "TRY_COMPILE with content requiring __PIC__ failed. ${OUTPUT}")
+endif()