[ruby-core:91824] [Ruby trunk Bug#15665] Cannot compile socket extension on Mojave

[franklinyu@hotmail.com]

Issue #15665 has been reported by franklinyu (Franklin Yu).

----------------------------------------
Bug #15665: Cannot compile socket extension on Mojave
https://bugs.ruby-lang.org/issues/15665

* Author: franklinyu (Franklin Yu)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: 2.6.0
* Backport: 2.4: UNKNOWN, 2.5: UNKNOWN, 2.6: UNKNOWN
----------------------------------------
The `mkmf.log` is reporting that it cannot find `netinet6/in6.h`. I guess that previously it’s finding it in `/usr/include`, but as we all know Apple removed that path since Mojave. On my machine the current path is:

```
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk/usr/include/
```

The C compiler seems fine if I’m using it stand-alone. I tried compiling following snippet:

```c
#include <netinet/in.h>
int main() {}
int t(struct in6_addr *addr) {
    return IN6_IS_ADDR_UNSPECIFIED(addr);
}
```

And it works, although I have no idea which header it included. I tried both compiling it with `cc test.c`, and with the actual command in `mkmf.log`:

```
clang -I../../.ext/include/x86_64-darwin18 -I../.././include -I../.././ext/socket -I../.. -I../../. -I/opt/local/include -D_XOPEN_SOURCE -D_DARWIN_C_SOURCE -D_DARWIN_UNLIMITED_SELECT -D_REENTRANT -Wall -Wextra -Wpedantic -Wshadow -pipe -D__APPLE_USE_RFC_3542 -Werror
```

Both works without any warning/error.

## note

This only happens to 2.6.0 and 2.6.1. Releases in 2.5.x or below are not affected.



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