zig c++ doesn't set the system include directories when input file is .h

[SpexGuy]

When compiling with zig c++, if the input file is .h instead of .cpp the system include directories are not set up properly. With a .cpp file:

-isystem C:\dev\zig\build-release\lib\zig\libcxx\include
-isystem C:\dev\zig\build-release\lib\zig\include
-isystem C:\dev\zig\build-release\lib\zig\libc\include\x86_64-windows-gnu
-isystem C:\dev\zig\build-release\lib\zig\libc\include\generic-mingw
-isystem C:\dev\zig\build-release\lib\zig\libc\include\x86_64-windows-any
-isystem C:\dev\zig\build-release\lib\zig\libc\include\any-windows-any

With a .h file:

-isystem C:\dev\zig\build-release\lib\zig\libcxx\include

If you're wondering "why would you ever want to do that?", the answer is that I have a tool that parses a preprocessed header. So it uses zig c++ -DSTUFF=FOO -E header.h and then goes through the output and parses it.

This works fine with clang (as long as -nostdinc is not specified).

[SpexGuy]

This is specifically on windows, when compiling with --target=native-native-gnu. The default target on windows is currently broken due to #5312

[wozeparrot]

This also happens on linux.