Cvpcb link process: add missing openmp lib (used by 3d viewer since rev 5544) in library list to link.

* Move perform_feature_checks() before GCC configuration to allow compiler
  option tests to be recognized during the first build when no cache file
  exists.

* auto-save was broken for all sub sheets with are in a sub-directory of the project directory. (reason wxFileName::Normalize(), used to create the absolute path  has issues with filenames starting by '$', perhaps due to env var which also start by $). the auto-save prefix is modified ( now AUTOSAVE_PREFIX_FILENAME which defines "_save_"  ) and defined only once.
* auto-save files from sub-sheets were not found due to the fact the path was not set (fixed by Blair Bonnet's patch)

Fix 2 coverity warnings.

The GCC/LLVM specific handling for adding -fvisibility options is later in
this file, so these options would only be added on the second invocation
without clearing the cache.

tools/edit_tool.cpp: remove limitation (GAL mode) to delete the last pad in a footprint (a footprint can have 0 pad, for instance a logo)

*  Use only double to store zoom levels in class BASE_SCREEN (int type is not working for eeschema or page layout editor) (this fix Bug #1450551, Assertion failed on wxRound() when zooming)
*  Fix crash in all apps but Pcbnew when a grid or a zoom was selected from the popup menu.
*  Modify default hotkeys for Next Grid and Previous Grid: the initial key was a dead key, not suitable as hotkey. the 'N' and Shift N keys are used instead (with a change for unfill zone hotkey)
*  Modify code in EDA_DRAW_FRAME::SetNextGrid() and EDA_DRAW_FRAME::SetPreviousGrid() to make them compatible with other editors than the board editor.