Enable alloca on mingw/cygwin builds:

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

CMakeLists.txt

@@ -855,9 +855,7 @@ if(SDL_LIBC)
       string(TOUPPER ${_FN} _UPPER)
       set(HAVE_${_UPPER} 1)
     endforeach()
-    if(NOT CYGWIN AND NOT MINGW)
-      set(HAVE_ALLOCA 1)
-    endif()
+    set(HAVE_ALLOCA 1)
     set(HAVE_M_PI 1)
     target_compile_definitions(sdl-build-options INTERFACE "-D_USE_MATH_DEFINES") # needed for M_PI
     set(STDC_HEADERS 1)

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