GNU sort isn't available on older macOS and some BSD systems

Don't prevent building entirely, just warn that we won't be able to find dynamic libraries in this case.

Fixes https://github.com/libsdl-org/SDL/pull/6338

configure

@@ -17955,7 +17955,8 @@ test -n "$SORT" || SORT="false"
 
 if ! "$SORT" -V </dev/null >/dev/null
 then :
-  as_fn_error $? "GNU sort(1) is required" "$LINENO" 5
+  { printf "%s\n" "$as_me:${as_lineno-$LINENO}: WARNING: GNU sort(1) is required to find dynamic libraries" >&5
+printf "%s\n" "$as_me: WARNING: GNU sort(1) is required to find dynamic libraries" >&2;}
 fi
 
 # Check whether --enable-largefile was given.

configure.ac

@@ -71,7 +71,7 @@ if [ test -z "$AWK" ]; then
 fi
 
 AC_CHECK_PROGS([SORT], [gsort sort], [false])
-AS_IF([! "$SORT" -V </dev/null >/dev/null], [AC_MSG_ERROR([GNU sort(1) is required])])
+AS_IF([! "$SORT" -V </dev/null >/dev/null], [AC_MSG_WARN([GNU sort(1) is required to find dynamic libraries])])
 
 dnl 64-bit file offsets if possible unless --disable-largefile is specified
 AC_SYS_LARGEFILE