fix build on ubuntu 16.04 (use cmake c++11 directive)

avoid the following error

In file included from /usr/include/c++/5/chrono:35:0,
from /home/user/tmp/dsdcc/timeutil.h:17,
from /home/user/tmp/dsdcc/dsd_decoder.cpp:19:
/usr/include/c++/5/bits/c++0x_warning.h:32:2: error: #error This file requires compiler and library support for the ISO C++ 2011 standard. This support must be enabled with the -std=c++11 or -std=gnu++11 compiler options.

CMakeLists.txt

@@ -1,3 +1,10 @@
+cmake_minimum_required(VERSION 3.1.0)
+
+# use c++11
+set(CMAKE_CXX_STANDARD 11)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+set(CMAKE_CXX_EXTENSIONS OFF)
+
 # use, i.e. don't skip the full RPATH for the build tree
 set(CMAKE_SKIP_BUILD_RPATH  FALSE)
 
@@ -12,7 +19,7 @@ SET(CMAKE_INSTALL_RPATH "${CMAKE_INSTALL_PREFIX}/lib")
 set(CMAKE_INSTALL_RPATH_USE_LINK_PATH TRUE)
 
 # Compiler flags.
-set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra -fmax-errors=10 -std=c++11 -O2 -ffast-math -ftree-vectorize ${EXTRA_FLAGS}")
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra -fmax-errors=10 -O2 -ffast-math -ftree-vectorize ${EXTRA_FLAGS}")
 
 list(APPEND CMAKE_MODULE_PATH ${CMAKE_SOURCE_DIR}/cmake/Modules)