[build] Improve visibility support of library function names.

* include/freetype/config/public-macros.h
(FT_PUBLIC_FUNCTION_ATTRIBUTE): New macro to tag functions as
public (and thus exportable).
(FT_EXPORT): Use it.

* include/freetype/config/compiler-macros.h
(FT_INTERNAL_FUNCTION_ATTRIBUTE): New macro to tag functions as
internal to the library (and thus hidden).  Note that on ELF
systems, all internal functions have hidden visibility, which avoids
the need to enforce this when invoking the compiler (e.g., with an
option like `-fvisibility=hidden').

(FT_FUNCTION_DECLARATION, FT_FUNCTION_DEFINITION): New base macros
to deal with C and C++ linkage issues at the same time.

(FT_LOCAL, FT_LOCAL_DEF, FT_LOCAL_ARRAY, FT_LOCAL_ARRAY_DEF,
FT_BASE, FT_BASE_DEF, FT_EXPORT_VAR, FT_BASE_CALLBACK,
FT_BASE_CALLBACK_DEF): Redefined using new macros.
3 files changed