allow the CMAKE_CXX_STANDARD could be set in the command line (#27)

* allow the CMAKE_CXX_STANDARD could be set in the command line, such as -DCMAKE_CXX_STANDARD=17.

* update the doc.

* WIP: avoid the hardcode of c++ standard in sniper-config.

CMakeLists.txt

@@ -35,7 +35,7 @@ if(WIN32)
 endif()
 
 # Compiler and Linker Settings
-set(CMAKE_CXX_STANDARD 11)
+set(CMAKE_CXX_STANDARD 11 CACHE STRING "")
 set(CMAKE_CXX_STANDARD_REQUIRED True)
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fPIC")
 if ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang")

README.md

@@ -41,6 +41,7 @@ There are more cmake options we can use while configuring the build:
 * `-DUSE_PYTHON=ON` : Build the project with Python support. A pure C++ version will be built if it is turned off. The default value is `ON`.
     * `-DUSE_PYTHON2=OFF` : Turn it on to use Python2, otherwise Python3 is used by default.
 * `-DUSE_ROOT=ON` : Build components that depend on ROOT. The default value is `ON`.
+* `-DCMAKE_CXX_STANDARD=11|14|17` : specify the C++ standard 11, 14, 17 or newer.
 
 Then build and install the result. Default build agent is `make` and you can call it directly.
 

cmake/sniper-config.in

@@ -79,7 +79,7 @@ while test $# -gt 0; do
         ;;
 
     --cflags)
-        echo "-std=c++11 -fPIC -I$includedir -I@Boost_INCLUDE_DIR@ -I@PYTHON_INCLUDE_DIRS@"
+        echo "-std=c++@CMAKE_CXX_STANDARD@ -fPIC -I$includedir -I@Boost_INCLUDE_DIR@ -I@PYTHON_INCLUDE_DIRS@"
         ;;
 
     --libdir)