Clean types.
configure.ac : Rename HAVE_LIBSNDFILE1 to HAVE_LIBSNDFILE.
src/question_dialogs.h : Add GNU gcc format printf checking.
src/question_dialogs.c : Use vsnprintf instead of snprintf where args
are passed as a va_list. Use fixes length array in sweep_perror_data
struct.
src/file_sndfile.c : Replace deprecated sf_error_str() with sf_strerror().
src/question_dialogs.[ch] : Make parameters 'const char*'.
src/file_sndfile.c : Replace deprecated sf_error_str() with sf_strerror().
Make it compile with current versions of tools and libs.

Autotool changes:
* Use AM_CPPFLAGS instead of INCLUDES in all the Makefile.am.

Code changes:
* GMutex in glib is now a struct which means we can allocate it within a
  struct. We then use g_mutex_init() instead of g_mutex_new() and
   g_mutex_free() instead of g_mutex_clear().
* Replace deprecated g_strncasecmp() with g_ascii_strncasecmp().
* Replace deprecated g_basename() with g_path_get_basename().
Merge pull request #3 from erikd/master

Bunch of fixes.