add SYSTEM to include to prevent warnings

[stingray-11]

This prevents compile warnings within cpp-httplib from showing up in an end user's build.

#1426

[yhirose]

@sum01 do you have any comment on this pull request? Thanks!

[sum01]

I've never actually seen SYSTEM used, so I wasn't sure of its purpose.

Looking it up, it doesn't seem like it really applies to this project, but maybe I'm wrong? To me, it seems like SYSTEM is meant for external includes, not internal ones. @stingray-11

[stingray-11]

It is for external includes, but this project itself is "external" for other projects that use it. Other projects might want to be able to enable extra compiler warnings that httplib does not yet pass, and use -Werror. Since httplib is a library I would guess this is the usual case.

I cannot find the Stack overflow post I was looking at before but this one on the cmake forum says "CMake expects the library author not the consumer to set the which include directories should be considered SYSTEM".

[sum01]

I guess that makes sense. Weird I've not seen others using it before though.

[yhirose]

Thanks for the contribution!