Whether c++ exceptions are enabled by default

[waruqi]

When c++ exceptions are not turned on, compiling some c++ code may result in c++ exception warnings, especially on msvc, for example:

If no flags are set, msvc disables c++ exceptions by default, unless the user sets /Ehsc, and then vs adds this flags by default.

In xmake, it is common to not add any flags whenever possible, but at the moment, given that most c++ projects require the use of exceptions, c++ exceptions are turned on by default in order to avoid msvc warnings.

xmake/xmake/rules/c++/xmake.lua

Lines 39 to 41 in 80af97d

	if target:is_plat("windows") and not target:get("exceptions") then
	target:set("exceptions", "cxx")
	end

But user can also disable it in own project. like this

set_exceptions("none")

This is the current default behaviour, do we need to keep the current default behaviour or disable c++ exceptions by default?

---

当前的版本默认会启用 c++ 异常，如果用户没有设置任何 c++ 异常 Flags，那么默认就会启用它，对于 msvc 会添加 /Ehsc flags。

当然用户可以显式通过 set_exceptions("none") 在自己的工程中禁用它。

xmake/xmake/rules/c++/xmake.lua

Lines 39 to 41 in 80af97d

	if target:is_plat("windows") and not target:get("exceptions") then
	target:set("exceptions", "cxx")
	end

我们是否需要修改这种行为，改成默认禁用 c++ 异常，不添加任何 flags，仅仅让用户自己去启用 c++ 异常。set_exceptions("cxx")

目前 vs 里面默认是开启的，并且如果不开异常的，编译一些 c++ 代码，会有警告提示。