compiler  warnings  v6  (lp:#1790859)

[Original report](https://bugs.launchpad.net/bugs/1790859) created by **Franck78 (fbourdonnec)**
```
Hello

here FYI the warnings during building of v6, revision September 4, 2018

gcc 5.4.0 on Ubuntu

make
...
[ 69%] Built target specctra_lexer_source_files
[ 69%] Generating pcbnew_wrap.cxx, pcbnew.py
/home/test/dev-kicad6/kicad/pcbnew/../include/common.h:379: Warning 317:
Specialization of non-template 'less'.
/home/test/dev-
kicad6/kicad/pcbnew/../include/geometry/shape_line_chain.h:67: Warning 325:
Nested struct not currently supported (INTERSECTION ignored)
/home/test/dev-
kicad6/kicad/pcbnew/../include/geometry/shape_line_chain.h:501: Warning
325: Nested struct not currently supported (compareOriginDistance ignored)
/home/test/dev-
kicad6/kicad/pcbnew/../include/geometry/shape_poly_set.h:115: Warning 325:
Nested class not currently supported (TRIANGULATED_POLYGON ignored)
/home/test/dev-
kicad6/kicad/pcbnew/../include/geometry/shape_poly_set.h:133: Warning 325:
Nested struct not currently supported (VERTEX_INDEX ignored)
/home/test/dev-
kicad6/kicad/pcbnew/../include/geometry/shape_poly_set.h:261: Warning 325:
Nested class not currently supported (ITERATOR_TEMPLATE ignored)
/home/test/dev-
kicad6/kicad/pcbnew/../include/geometry/shape_poly_set.h:407: Warning 325:
Nested class not currently supported (SEGMENT_ITERATOR_TEMPLATE ignored)
io_mgr.h:123: Warning 325: Nested class not currently supported
(PLUGIN_REGISTRY ignored)
io_mgr.h:138: Warning 325: Nested struct not currently supported
(REGISTER_PLUGIN ignored)
class_zone.h:778: Warning 325: Nested union not currently supported
(WX_VECTOR_CONVERTER ignored)
class_module.h:98: Warning 325: Nested struct not currently supported
(VECTOR3D ignored)
/home/test/dev-kicad6/kicad/pcbnew/../include/colors_design_settings.h:48:
Warning 401: Nothing known about base class 'SETTINGS'. Ignored.
/home/test/dev-kicad6/kicad/pcbnew/../include/eda_text.h:45: Warning 401:
Nothing known about base class 'std::mutex'. Ignored.
connectivity_data.h:80: Warning 401: Nothing known about base class
'LOCKABLE'. Ignored.
class_board.h:50: Warning 315: Nothing known about 'std::unique_ptr'.
/home/test/dev-kicad6/kicad/pcbnew/../include/math/vector2d.h:126: Warning
503: Can't wrap 'operator wxPoint' unless renamed to a valid identifier.
/home/test/dev-kicad6/kicad/pcbnew/../include/utf8.h:187: Warning 503:
Can't wrap 'operator const std::string&' unless renamed to a valid
identifier.
swig_import_helper fixed for /home/test/dev-
kicad6/kicad/build/pcbnew/pcbnew.py


Scanning dependencies of target pcbnew_kiface_objects
[ 84%] Building CXX object
pcbnew/CMakeFiles/pcbnew_kiface_objects.dir/swig/python_scripting.cpp.o
[ 84%] Built target pcbnew_kiface_objects
Scanning dependencies of target pnsrouter
[ 84%] Building CXX object
pcbnew/router/CMakeFiles/pnsrouter.dir/time_limit.cpp.o
[ 84%] Building CXX object
pcbnew/router/CMakeFiles/pnsrouter.dir/pns_kicad_iface.cpp.o
/home/test/dev-kicad6/kicad/pcbnew/router/pns_kicad_iface.cpp: In member
function ‘bool PNS_KICAD_IFACE::syncZone(PNS::NODE*, ZONE_CONTAINER*)’:
/home/test/dev-kicad6/kicad/pcbnew/router/pns_kicad_iface.cpp:824:31:
warning: comparison between signed and unsigned integer expressions
[-Wsign-compare]
             for( int i = 0; i < tri->GetTriangleCount(); i++)
                               ^
[ 84%] Building CXX object
pcbnew/router/CMakeFiles/pnsrouter.dir/pns_algo_base.cpp.o
[ 84%] Building CXX object
pcbnew/router/CMakeFiles/pnsrouter.dir/pns_diff_pair.cpp.o
```

Story points: 6.0