only use -flto when supported, quenched policy warnings

CMakeLists.txt

@@ -27,14 +27,23 @@ set(Python_ADDITIONAL_VERSIONS 3.4 3.5 3.6)
 find_package(PythonLibs ${PYBIND11_PYTHON_VERSION} REQUIRED)
 find_package(PythonInterp ${PYBIND11_PYTHON_VERSION} REQUIRED)
 
-if ("${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang" OR "${CMAKE_CXX_COMPILER_ID}" MATCHES "GNU")
+include(CheckCXXCompilerFlag)
+
+if (CMAKE_CXX_COMPILER_ID MATCHES "Clang" OR CMAKE_CXX_COMPILER_ID MATCHES "GNU")
   # Enable C++11 mode on C++ / Clang
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
 
   # Enable link time optimization and set the default symbol
   # visibility to hidden (very important to obtain small binaries)
   if (NOT ${U_CMAKE_BUILD_TYPE} MATCHES DEBUG)
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fvisibility=hidden -flto")
+    # Default symbol visibility
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fvisibility=hidden")
+
+    # Check for Link Time Optimization support
+    CHECK_CXX_COMPILER_FLAG("-flto" HAS_LTO_FLAG)
+    if (HAS_LTO_FLAG)
+      set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -flto")
+    endif()
   endif()
 endif()