Use compiler symbol visiblity features to hide internal functions

Previously we relied on libtool's -export-symbols-regex feature, but we
are hoping to drop GNU Autotools and GNU Libtool in soon in favour of
Meson.

Meson doesn't have an equivalent feature, instead the advice is to
control symbol visibility at compile time.

The approach taken in this patch is based on Pango's build system. Pango
tells the compiler to hide symbols by default (if possible), and then defines
a _PANGO_EXTERN variable at compile time which marks a single symbol as
public. In Pango's case there is then further machinary to hide symbols
based on deprecation policies but I have not copied that here, instead I
used _LIBMEDIAART_EXTERN directly.

If a compiler doesn't support hiding symbols then the library we build
makes all symbols available, which is exactly what would happen before
on platforms where the libtool didn't have an implementation for
-export-symbols-regex.

See also:

  http://mesonbuild.com/FAQ.html#how-do-i-do-the-equivalent-of-libtools-exportsymbol-and-exportregex
  https://git.gnome.org/browse/pango/
  https://git.gnome.org/browse/pango/tree/pango/pango-version-macros.h
  https://gcc.gnu.org/wiki/Visibility
  https://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html#Code-Gen-Options (-fvisibility)

https://bugzilla.gnome.org/show_bug.cgi?id=783562

1 files changed, 6 insertions, 0 deletions

diff --git a/libmediaart/extractgeneric.h b/libmediaart/extractgeneric.h
index 8fa114b..b08fe2c 100644
--- a/libmediaart/extractgeneric.h
+++ b/libmediaart/extractgeneric.h
@@ -25,18 +25,24 @@
 
 #include <glib.h>
 
+#include "mediaart-macros.h"
+
 #if !defined (__LIBMEDIAART_INSIDE__) && !defined (LIBMEDIAART_COMPILATION)
 #error "Only <libmediaart/mediaart.h> must be included directly."
 #endif
 
 G_BEGIN_DECLS
 
+_LIBMEDIAART_EXTERN
 void      media_art_plugin_init     (gint                  max_width);
+_LIBMEDIAART_EXTERN
 void      media_art_plugin_shutdown (void);
 
+_LIBMEDIAART_EXTERN
 gboolean  media_art_file_to_jpeg    (const gchar          *filename,
                                      const gchar          *target,
                                      GError              **error);
+_LIBMEDIAART_EXTERN
 gboolean  media_art_buffer_to_jpeg  (const unsigned char  *buffer,
                                      size_t                len,
                                      const gchar          *buffer_mime,