Fix LTO support for cross-compilation.

When cross-compiling, the ar and ranlib to be used for LTO are prefixed
by the cross-tuple. gcc-ar and gcc-ranlib may not exist. Cfr.
http://autobuild.buildroot.net/results/f3c/f3c48da3a9706cd366c0e0a96c3cd0ff959f2a78/

Therefore, search for an appropriate lto-ar and lto-ranlib before
enabling LTO.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

CMakeLists.txt

@@ -39,9 +39,15 @@ if (NOT ${U_CMAKE_BUILD_TYPE} MATCHES DEBUG)
     CHECK_CXX_COMPILER_FLAG("-flto" HAS_LTO_FLAG)
 
     if (HAS_LTO_FLAG)
-        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -flto")
-        set(CMAKE_AR gcc-ar)
-        set(CMAKE_RANLIB gcc-ranlib)
+        find_program(LTO_AR NAMES "${CMAKE_C_COMPILER}-ar" gcc-ar)
+        find_program(LTO_RANLIB NAMES "${CMAKE_C_COMPILER}-ranlib" gcc-ranlib)
+        if (LTO_AR AND LTO_RANLIB)
+            set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -flto")
+            set(CMAKE_AR "${LTO_AR}")
+            set(CMAKE_RANLIB "${LTO_RANLIB}")
+        else()
+            message(STATUS "gcc-ar or gcc-ranlib not found, disabling LTO")
+        endif()
     endif()
 endif()