Re-enable switch to C-locale when searching for C++ includes (ROOT-10751)
Re-enable switch to C-locale when searching for C++ includes (ROOT-10751)

Commit df0b689 simplified the search for the C++ include directories and
tried to use a locale independent regex.

However this regex doesn't work for all locales, for example in German
the string in the output is not

```
#include <...> search starts here:
```

but the word order is switched and we have

```
Suche für »#include <...>« beginnt hier:
```

As such I propose to go back to C-locale for this query to be completely
safe against weird locale settings.

root-mirror/cling,root-mirror/cling,root-mirror/cling,root-mirror/cling,root-mirror/cling,root-mirror/cling