Update SDL to 2.0.18

libs/SDL2/autogen.sh

@@ -1,5 +1,5 @@
 #!/bin/sh
-#
+
 echo "Generating build information using autoconf"
 echo "This may take a while ..."
 
@@ -9,15 +9,15 @@ cd "$srcdir"
 
 # Regenerate configuration files
 cat acinclude/* >aclocal.m4
-found=false
-for autoconf in autoconf autoconf259 autoconf-2.59
-do if which $autoconf >/dev/null 2>&1; then $autoconf && found=true; break; fi
-done
-if test x$found = xfalse; then
-    echo "Couldn't find autoconf, aborting"
-    exit 1
+
+if test "$AUTOCONF"x = x; then
+  AUTOCONF=autoconf
 fi
+
+$AUTOCONF || exit 1
+rm aclocal.m4
+rm -rf autom4te.cache
+
 (cd test; sh autogen.sh)
 
-# Run configure for this platform
 echo "Now you are ready to run ./configure"