argp: port new 'inline' approach to Sun C 5.12 + Solaris 10

argp: port new 'inline' approach to Sun C 5.12 + Solaris 10

On this platform, we are not optimizing but we are using
the substitute for extern inlines, so compile as if
C99-style extern inline, or a substitute, is available.
* lib/argp-fmtstream.h (argp_fmtstream_set_lmargin)
(__argp_fmtstream_set_lmargin, argp_fmtstream_set_rmargin)
(__argp_fmtstream_set_rmargin, argp_fmtstream_set_wmargin)
(__argp_fmtstream_set_wmargin, argp_fmtstream_point)
(__argp_fmtstream_point) [!_LIBC && !__OPTIMIZE__]:
Declare as ARGP_FS_EI, not as extern.
* lib/argp.h (argp_usage, __argp_usage, _option_is_short)
(__option_is_short, _option_is_end, __option_is_end)
[!_LIBC && __USE_EXTERN_INLINES]:
Declare as ARGP_EI, not as extern.