build: various build fixes for macOS

This doesn't make profiling useful in any way, but it does get things
to the point where I can actually open a capture file. And it would
be nice if we could progress to the point of loading capture files
(with correct data in-tact) and this helps us move down that path.

To really do that correctly, we should make some of the widgetry
disabled when it isn't useful. We also need to ensure that we add extra
decoding information to capture files during shutdown so that any
platform can read it back. This would also help the situation of
running and reading on separate architectures.

src/libsysprof/elfparser.c

@@ -15,9 +15,14 @@
  * along with this program; if not, write to the Free Software
  * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
  */
+
 #include <stdlib.h>
 #include <string.h>
-#include <elf.h>
+#ifdef __APPLE__
+# include <libelf.h>
+#else
+# include <elf.h>
+#endif
 #include <sys/mman.h>
 
 #include "demangle.h"