Plugins: Require a C++11 compiler + enable C++11 features

Related documentation:
https://www.gnu.org/software/autoconf-archive/ax_cxx_compile_stdcxx.html

Author: hartwork

libvisual-plugins/configure.ac

@@ -46,6 +46,7 @@ AC_PROG_CXX
 dnl Check for typedefs, structures, and compiler characteristics.
 AC_C_CONST
 AC_C_INLINE
+AX_CXX_COMPILE_STDCXX_11([noext], [mandatory])
 
 dnl 'Standard' header files.
 AC_CHECK_HEADERS([fcntl.h unistd.h stdlib.h string.h sys/time.h])