Make lto detection more robust and not resetting cxx flags when lto fails.

This refines the last commit of parallelizing lto.

Discussion: this is ugly as hell, dispatching 1/ on the availability of
the `-flto` flag, then 2/ on the compiler since GCC allows `-flto=n`
whereas Clang for example does not.

I tried setting the CMake property `INTERPROCEDURAL_OPTIMIZATION`,
without any effect. All I could see was some lto related utilities in
the cmake debug output, but not in the actual compiler or linker
invocation.

This would eliminate the need for our hacks, with 1/ using an option
`WITH_LTO` setting `ON` by default, and based on this value setting the
`INTERPROCEDURAL_OPTIMIZATION` flag with CMake doing the actual work of
selecting the best LTO method on the target platform.

By the way, this also fixes a bug where we reset the `CMAKE_CXX_FLAGS`
to a variable that was never defined, resulting in setting the flags to
an empty string. Yay CMake, as usual.

References:

- http://www.cmake.org/cmake/help/v3.0/prop_tgt/INTERPROCEDURAL_OPTIMIZATION.html

CMakeLists.txt

@@ -124,12 +124,17 @@ endif()
 if(CMAKE_BUILD_TYPE MATCHES Release)
   message(STATUS "Configuring OSRM in release mode")
   # Check if LTO is available
+  check_cxx_compiler_flag("-flto" LTO_AVAILABLE)
+  if(LTO_AVAILABLE)
+    set(OLD_CXX_FLAGS ${CMAKE_CXX_FLAGS})
+    # GCC in addition allows parallelizing LTO
+    if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
       include(ProcessorCount)
       ProcessorCount(NPROC)
-  set(LTO_FLAGS "")
-  check_cxx_compiler_flag("-flto=${NPROC}" LTO_AVAILABLE)
-  if(LTO_AVAILABLE)
       set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -flto=${NPROC}")
+    else()
+      set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -flto")
+    endif()
     set(CHECK_LTO_SRC "int main(){return 0;}")
     check_cxx_source_compiles("${CHECK_LTO_SRC}" LTO_WORKS)
     if(LTO_WORKS)