Cling header search doesn't work with some languages

The regex for parsing the header paths in the`interpreter/cling/lib/Interpreter/CMakeLists.txt` file doesn't work with localized versions of GCC (e.g. LANG!=C).

In our case we have a user that has this output when running gcc:

```
la búsqueda de #include "..." inicia aquí:
la búsqueda de #include <...> inicia aquí:
/usr/lib/gcc/x86_64-pc-linux-gnu/7.1.1/../../../../include/c++/7.1.1
/usr/lib/gcc/x86_64-pc-linux-gnu/7.1.1/../../../../include/c++/7.1.1/x86_64-pc-linux-gnu
/usr/lib/gcc/x86_64-pc-linux-gnu/7.1.1/../../../../include/c++/7.1.1/backward
/usr/lib/gcc/x86_64-pc-linux-gnu/7.1.1/include
/usr/local/include
/usr/lib/gcc/x86_64-pc-linux-gnu/7.1.1/include-fixed
/usr/include
```

Which won't be recognized by this regex:
`COMMAND awk "/<sup>#include </,/^End of search/{if (!/</sup>#include </ && !/^End of search/){ print }}"`

We should force set LANG=C in these situations.