Add new public macros for GCC attributes.
* src/gpg-error.h.in (GPGRT_GCC_VERSION): New. (GPGRT_ATTR_NORETURN, GPGRT_ATTR_PRINTF, GPGRT_ATTR_NR_PRINTF): New. (GPGRT_ATTR_FORMAT_ARG, GPGRT_ATTR_SENTINEL): New. (GPGRT_ATTR_USED, GPGRT_ATTR_UNUSED, GPGRT_ATTR_DEPRECATED): New. (GPGRT_ATTR_PURE, GPGRT_ATTR_MALLOC): New. (GPGRT_HAVE_MACRO_FUNCTION, GPGRT_HAVE_PRAGMA_GCC_PUSH): New. (_GPGRT_GCC_A_PRINTF): Replace GPGRT_ATTR_PRINTF.