Enable alloca on mingw/cygwin builds:

This effectively reverts commit 391d73e37b26614e2e343ca8e9d4334e07380896
Fixes https://github.com/libsdl-org/SDL/issues/5234

configure.ac

@@ -318,13 +318,7 @@ dnl Check for typedefs, structures, etc.
 dnl Check for defines
     AC_CHECK_DEFINE(M_PI, math.h)
 
-    case "$host" in
-    *-*-cygwin* | *-*-mingw*)
-        ;;
-    *)
-        AC_FUNC_ALLOCA
-        ;;
-    esac
+    AC_FUNC_ALLOCA
 
 dnl Checks for library functions.
     AC_FUNC_MEMCMP