Explicitly hide static symbols

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;
only the latter are exported, the former are hidden.

In addition, -fvisibility=hidden is moved to CFLAGS for libzstd-mt (as
done previously for libzstd).

Signed-off-by: Stephen Kitt <[email protected]>

lib/Makefile

@@ -330,7 +330,8 @@ include $(wildcard $(DEPFILES))
 # Special case : building library in single-thread mode _and_ without zstdmt_compress.c
 ZSTDMT_FILES = compress/zstdmt_compress.c
 ZSTD_NOMT_FILES = $(filter-out $(ZSTDMT_FILES),$(ZSTD_FILES))
-libzstd-nomt: LDFLAGS += -shared -fPIC -fvisibility=hidden
+libzstd-nomt: CFLAGS += -fvisibility=hidden
+libzstd-nomt: LDFLAGS += -shared -fPIC
 libzstd-nomt: $(ZSTD_NOMT_FILES)
 	@echo compiling single-thread dynamic library $(LIBVER)
 	@echo files : $(ZSTD_NOMT_FILES)

lib/compress/zstd_compress.c

@@ -1039,7 +1039,7 @@ size_t ZSTD_CCtx_loadDictionary_advanced(
     return 0;
 }
 
-ZSTDLIB_API size_t ZSTD_CCtx_loadDictionary_byReference(
+ZSTDLIB_STATIC_API size_t ZSTD_CCtx_loadDictionary_byReference(
       ZSTD_CCtx* cctx, const void* dict, size_t dictSize)
 {
     return ZSTD_CCtx_loadDictionary_advanced(
@@ -3794,7 +3794,7 @@ ZSTD_CDict* ZSTD_createCDict_advanced(const void* dictBuffer, size_t dictSize,
         &cctxParams, customMem);
 }
 
-ZSTDLIB_API ZSTD_CDict* ZSTD_createCDict_advanced2(
+ZSTDLIB_STATIC_API ZSTD_CDict* ZSTD_createCDict_advanced2(
         const void* dict, size_t dictSize,
         ZSTD_dictLoadMethod_e dictLoadMethod,
         ZSTD_dictContentType_e dictContentType,