snippet/c++defs: Simplify _GL_CXXALIAS_* macros. * build-aux/snippet/c++defs.h [__cplusplus && GNULIB_NAMESPACE] (_GL_CXXALIAS_RPL_1, _GL_CXXALIAS_RPL_CAST_1, _GL_CXXALIAS_SYS, _GL_CXXALIAS_SYS_CAST, _GL_CXXALIAS_SYS_CAST2): Inline and remove member function 'rpl ()' of the wrapper struct.