From faaee1cbbd7ad5ac5c99a6f1fbb0562f2c3f548c Mon Sep 17 00:00:00 2001
From: Wenzel Jakob <wenzel@inf.ethz.ch>
Date: Mon, 30 Nov 2015 10:17:01 +0100
Subject: [PATCH] only use -flto when supported, quenched policy warnings

---
 CMakeLists.txt | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 2fa32e81a..c77ab8f7c 100644
--- a/CMakeLists.txt
+++ b/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()