Elevate C4013 to level 1

C4013 warns about undefined functions[1], but for some reason is only a
level 3 warning, and such suppressed by default (usually there will be
link error afterwards, though).  It appears to be sensible to elevate
the warning to level 1, so it will be converted to a compile error.

[1] <https://learn.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warning-level-3-c4013>

Author: cmb69

.github/scripts/windows/build_task.bat

@@ -31,7 +31,7 @@ if %errorlevel% neq 0 exit /b 3
 if "%THREAD_SAFE%" equ "0" set ADD_CONF=%ADD_CONF% --disable-zts
 if "%INTRINSICS%" neq "" set ADD_CONF=%ADD_CONF% --enable-native-intrinsics=%INTRINSICS%
 
-set CFLAGS=/W1 /WX
+set CFLAGS=/W1 /WX /w14013
 
 cmd /c configure.bat ^
 	--enable-snapshot-build ^