Compiler support for default template arguments

- Using default template arguments in function templates requires C++11
  compiler support. The configure script checks and adds the necessary
  options to CXXFLAGS.
- Fix default template argument repeated at function definition.
- Fix an error in m4/ax_check_quad_precision.m4 which add Intel compiler
  flags for __float128
- Include 'cstdlib' for size_t