Preserve hidden symbol visibility for static builds with GCC-compatible compilers

[realwangzhichao]

Problem

In the __GNUC__ >= 4 branch, TINYXML2_LIB is unconditionally defined as
__attribute__((visibility("default"))).

This overrides the hidden-by-default visibility configured by CMake, even when
tinyxml2 is built as a static library. When the static archive is linked as a
private dependency into a shared library, tinyxml2 symbols can consequently
appear in that shared library's dynamic symbol table.

This may cause symbol interposition when multiple versions of tinyxml2 are
loaded into the same process.

Change

Mirror the existing MSVC export/import logic in the __GNUC__ branch:

• Use visibility("default") when TINYXML2_EXPORT is defined.
• Use visibility("default") when TINYXML2_IMPORT is defined.
• Otherwise, leave TINYXML2_LIB undecorated so static builds preserve the
  configured hidden visibility.

Shared-library exports and imports remain unchanged.

Testing

Tested with GCC 13.3:

• Static Release build and ctest: passed.
• Shared Release build and ctest: passed.
• Linked the PIC static archive into a test shared object with --whole-archive
  and verified with nm -D that no tinyxml2:: symbols were exported.
• Verified with nm -D that the shared tinyxml2 library still exports its
  public symbols.