Make it possible to specify IncludeDir as system include path

[christian-schilling]

This would be usefull to silence warnings generated for library headers.
See: https://gcc.gnu.org/onlinedocs/cpp/System-Headers.html

[christian-schilling]

Maybe it should be the default behaviour to use -isystem instead of -I if the IncludeDir was brought in by
a Dependency, at least for GCC.
This should only apply to dependencies from other projects, not to dependencies on a different config in the same Project.meta.

[aschaal]

Automatic does not work. Problem here is: "All directories named by -isystem are searched after all directories named by -I, no matter what their order was on the command line." (see your linked spec)

Which means that e.g.

ExecutableConfig test {
IncludeDir x
Dependency y, config: mock
IncludeDir os/include
}

results in: -I -isystem -I<os/include>
Imagine y inherits some mock folders. This would probaly break the build, because the include folders defined in y are moved to the end.

Sure you could manually deactivate this feature, but imho opt-in should be used not opt-out.

---> I don't see any problem for manual tagging a system include directory (opt-in), e.g.

IncludeDir "include/os", system: true

Okay?

[aschaal]

Aadded to 2.25.0
IncludeDir "include/os", system: true
A system include overrules a normal include.