Use libsdl2 instead of libsdl 1.2, which is dead upstream.

Luckily the API that mpg123 use didn't change so this is a trivial change.

Upstream-Status: Submitted [https://sourceforge.net/p/mpg123/bugs/268/]
Signed-off-by: Ross Burton <ross.burton@intel.com>

diff --git a/configure.ac b/configure.ac
index 6216d4f..b8d3344 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1694 +1694 @@ do
-			PKG_CHECK_MODULES(SDL, sdl, output_modules="$output_modules sdl" HAVE_SDL="yes", HAVE_SDL="no" check_failed=yes)
+			PKG_CHECK_MODULES(SDL, sdl2, output_modules="$output_modules sdl" HAVE_SDL="yes", HAVE_SDL="no" check_failed=yes)