Frkaya/gcc fix 052919 (#24845)

Frkaya/gcc fix 052919 (#24845)

* fix compilation with gcc7.x

In file included from src/inc/sstring.h:1029:0,
                 from src/inc/ex.h:19,
                 from src/inc/stgpool.h:28,
                 from src/inc/../md/inc/metamodel.h:18,
                 from src/inc/../md/inc/metamodelro.h:19,
                 from src/inc/metadata.h:17,
                 from src/inc/corpriv.h:21,
                 from src/md/compiler/stdafx.h:20,
                 from src/md/compiler/mdutil.cpp:12:
src/inc/sstring.inl: In member function ‘const WCHAR* SString::GetUnicode() const’:
src/inc/sstring.inl:650:5: error: nonnull argument ‘this’ compared to NULL [-Werror=nonnull-compare]
     if (this == NULL)
     ^~

add version check

* Use the abstracted version of __nodebug__

* cc1: warning: command line option ‘-fpermissive’ is valid for C++/ObjC++ but not for C