Distinguish static symbols, allow hiding them

Distinguish static symbols, allow hiding them

Even with -fvisibility=hidden added to CFLAGS, any symbol which is
given a default visibility attribute ends up exported in the dynamic
library. This happens through zstd_internal.h which defines
..._STATIC_LINKING_ONLY before including various header files, and is
included for example in lib/common/pool.c.

To avoid this, this patch distinguishes static and non-static APIs, by
using ZSTDLIB_API only for the latter, and introducing
ZSTDLIB_STATIC_API for the former. For now, both are exported, but
non-static APIs can be hidden by overriding the definition
ZSTDLIB_STATIC_API. lib/Makefile is modified to allow this using

	make CPPFLAGS_DYNLIB=-DZSTDLIB_STATIC_API=ZSTDLIB_HIDDEN

In addition, API declarations are dropped from zstd_compress.c (they
aren't needed there).

Signed-off-by: Stephen Kitt <steve@sk2.org>