Bump CMake requirement to 3.1 and fix code which requires 3.2+

CMake supports CXX_STANDARD target property (and therefore
automatic -std=c++11) since 3.1

Thanks to noteness for the report

CMakeLists.txt

@@ -14,7 +14,7 @@
 # limitations under the License.
 #
 
-cmake_minimum_required(VERSION 3.0)
+cmake_minimum_required(VERSION 3.1)
 project(ZNC VERSION 1.7.0)
 set(ZNC_VERSION 1.7.x)
 set(append_git_version true)
@@ -332,6 +332,4 @@ message("")
 # TODO
 # ====
 #
-# readme: advice about cmake --system-information
 # remove old configure.ac and Makefile.in
-# consider mininum cmake version

src/CMakeLists.txt

@@ -14,6 +14,11 @@
 # limitations under the License.
 #
 
+if(CMAKE_VERSION VERSION_LESS 3.2)
+	# Since 3.2 it does this automatically from BYPRODUCTS
+	set_source_files_properties("versionc.cpp" PROPERTIES GENERATED true)
+endif()
+
 znc_add_library(znclib ${lib_type} "ZNCString.cpp" "znc.cpp" "IRCNetwork.cpp"
 	"IRCSock.cpp" "Client.cpp" "Chan.cpp" "Nick.cpp" "Server.cpp"
 	"Modules.cpp" "MD5.cpp" "Buffer.cpp" "Utils.cpp" "FileUtils.cpp"

znc-buildmod.cmake.in

@@ -56,7 +56,7 @@ args = parser.parse_args()
 
 with tempfile.TemporaryDirectory() as cmdir:
     with open(os.path.join(cmdir, 'CMakeLists.txt'), 'w') as cm:
-        print('cmake_minimum_required(VERSION 3.0)', file=cm)
+        print('cmake_minimum_required(VERSION 3.1)', file=cm)
         print('project(ExternalModules)', file=cm)
         print('find_package(ZNC @ZNC_VERSION_MAJOR@.@ZNC_VERSION_MINOR@ PATHS '
                 '@CMAKE_INSTALL_FULL_DATADIR@/znc)', file=cm)