find_clang.cr, function parse_clang_output takes only first include path while skipping others

[ZaWertun]

For example I have this flags (https://github.com/Papierkorb/bindgen/blob/master/clang/find_clang.cr#L380), I'll include only -internal-isystem part:

...
 "-internal-isystem",
 "/usr/bin/../lib/gcc/x86_64-redhat-linux/10/../../../../include/c++/10",
 "/usr/bin/../lib/gcc/x86_64-redhat-linux/10/../../../../include/c++/10/x86_64-redhat-linux",
 "/usr/bin/../lib/gcc/x86_64-redhat-linux/10/../../../../include/c++/10/backward",
 "/usr/local/include",
 "/usr/lib64/clang/10.0.0/include",
 "-internal-externc-isystem",
...

And this function returns only first one dir - /usr/include/c++/10, skipping the /usr/local/include, /usr/lib64/clang/10.0.0/include and others.
And because I don't have /usr/lib64/clang/10.0.0/include in include paths defined, later I see this error running crystal spec:

In file included from /tmp/.kMdBOGbindgen:1:
In file included from /home/zawertun/tmp/bindgen@Papierkorb/spec/integration/c_wrapper.cpp:1:
In file included from /usr/lib/gcc/x86_64-redhat-linux/10/../../../../include/c++/10/cstdlib:75:
/usr/include/stdlib.h:31:10: fatal error: 'stddef.h' file not found
#include <stddef.h>
         ^~~~~~~~~~

File stddef.h can be found in the /usr/lib64/clang/10.0.0/include dir, if I add this include manually - crystal spec finishes without errors.

[docelic]

Will check what's going on.

[ZaWertun]

It just takes next item after -internal-isystem.

when "-internal-isystem"
      system_include_dirs << flags[index + 1]
      index += 1

I'll create PR.

[docelic]

Merged, thanks! I guess further improvements to this (e.g. possibly using llvm-config to get these flags instead of clang++) can be done through #36 at some later point.