Explicitly set visibility of public API functions when gcc is used

Explicitly set visibility of public API functions when gcc is used

Windows builds already limit exporting of functions to those marked
with LZ4LIB_API tag.  The same behaviour could be achieved on other
platforms when a relatively fresh gcc is used.

This change assigns public visibility to all symbols marked with
LZ4LIB_API tag.  When the library is built in -fvisibility=hidden
mode, only these marked symbols will be exported.