when LTO is available, build with LTO

Fixes: #140

CMakeLists.txt

@@ -123,12 +123,37 @@ install(FILES ${PROJECT_BINARY_DIR}/cbor/configuration.h DESTINATION include/cbo
 # Make the header visible at compile time
 include_directories(${PROJECT_BINARY_DIR})
 
+# CMake >= 3.9.0 enables LTO for GCC and Clang with INTERPROCEDURAL_OPTIMIZATION
+# Policy CMP0069 enables this behavior when we set the minimum CMake version < 3.9.0
+# Checking for LTO support before setting INTERPROCEDURAL_OPTIMIZATION is mandatory with CMP0069 set to NEW.
+set(use_lto FALSE)
+if(${CMAKE_VERSION} VERSION_GREATER "3.9.0" OR ${CMAKE_VERSION} VERSION_EQUAL "3.9.0")
+    cmake_policy(SET CMP0069 NEW)
+    # Require LTO support to build libcbor with newer CMake versions
+    include(CheckIPOSupported)
+    check_ipo_supported(RESULT use_lto)
+endif(${CMAKE_VERSION} VERSION_GREATER "3.9.0" OR ${CMAKE_VERSION} VERSION_EQUAL "3.9.0")
+if(use_lto)
+    message(STATUS "LTO is enabled")
+else()
+    message(STATUS "LTO is not enabled")
+endif(use_lto)
+
 subdirs(src)
+if(use_lto)
+    set_property(DIRECTORY src PROPERTY INTERPROCEDURAL_OPTIMIZATION TRUE)
+endif(use_lto)
 
 if (WITH_TESTS)
     subdirs(test)
+    if(use_lto)
+        set_property(DIRECTORY test PROPERTY INTERPROCEDURAL_OPTIMIZATION TRUE)
+    endif(use_lto)
 endif (WITH_TESTS)
 
 if (WITH_EXAMPLES)
-     subdirs(examples)
+    subdirs(examples)
+    if(use_lto)
+        set_property(DIRECTORY examples PROPERTY INTERPROCEDURAL_OPTIMIZATION TRUE)
+    endif(use_lto)
 endif (WITH_EXAMPLES)