Generate PYTHONPATH from CMake information

The CMake scripts calculate the install path for python scripts by
  distutils.sysconfig.get_python_lib(
    plat_specific=0,
    standard_lib=0,
    prefix=''
  )
which generates a string in line with
  lib/python2.7/site-packages

This string ends up in the CMake variable PYTHON_DEST and is used by the
install step as destination directory for pcbnew.py.

There has been a hard-coded assumption on the content of that string in
PcbNew which is not compatible with FreeBSD 11.1.

This commit eliminates the hard-coded assumption in PcbNew and reuses
the install path as known by CMake.

Fixes: lp:1777921
* https://bugs.launchpad.net/kicad/+bug/1777921

CMakeModules/config.h.cmake

@@ -72,4 +72,7 @@
 #define KIFACE_SUFFIX                   "@KIFACE_SUFFIX@"
 #define KIFACE_PREFIX                   "@KIFACE_PREFIX@"
 
+/// Allows scripts install directory to be referenced by the program code.
+#define PYTHON_DEST                     "@PYTHON_DEST@"
+
 #endif  // CONFIG_H_

pcbnew/pcbnew.cpp

@@ -263,10 +263,10 @@ static bool scriptingSetup()
     wxSetEnv( "PYTHONPATH", pypath );
 
 #else
-    // Linux-specific setup
     wxString pypath;
 
-    pypath = Pgm().GetExecutablePath() + wxT( "../lib/python2.7/dist-packages" );
+    // PYTHON_DEST is the scripts install dir as determined by the build system.
+    pypath = Pgm().GetExecutablePath() + wxT( "../" PYTHON_DEST );
 
     if( !wxIsEmpty( wxGetenv( wxT( "PYTHONPATH" ) ) ) )
         pypath = wxString( wxGetenv( wxT( "PYTHONPATH" ) ) ) + wxT( ":" ) + pypath;