Fix some g++ warnings "has a different exception specifier". * m4/gnulib-common.m4 (gl_COMMON_BODY): Make _GL_ATTRIBUTE_NOTHROW effective in C++ mode. * lib/attribute.h (ATTRIBUTE_NOTHROW): Add a comment. * lib/malloc.in.h (_GL_ATTRIBUTE_NOTHROW): Provide fallback definition. (memalign): Invoke _GL_FUNCDECL_SYS with _GL_ATTRIBUTE_NOTHROW. * lib/stdio.in.h (_GL_ATTRIBUTE_NOTHROW): Provide fallback definition. (fdopen): Invoke _GL_FUNCDECL_SYS with _GL_ATTRIBUTE_NOTHROW. * lib/stdlib.in.h (_GL_ATTRIBUTE_NOTHROW): Provide fallback definition. (aligned_alloc, calloc, canonicalize_file_name, malloc, realloc): Invoke _GL_FUNCDECL_SYS with _GL_ATTRIBUTE_NOTHROW. * lib/string.in.h (_GL_ATTRIBUTE_NOTHROW): Provide fallback definition. (strdup, strndup): Invoke _GL_FUNCDECL_SYS with _GL_ATTRIBUTE_NOTHROW. * lib/sys_stat.in.h (_GL_ATTRIBUTE_NOTHROW): Provide fallback definition. (getumask): Invoke _GL_FUNCDECL_SYS with _GL_ATTRIBUTE_NOTHROW. * lib/wchar.in.h (_GL_ATTRIBUTE_NOTHROW): Provide fallback definition. (wcsdup): Invoke _GL_FUNCDECL_SYS with _GL_ATTRIBUTE_NOTHROW.