Use KICAD_DATA to find stock libraries on Linux

Use the KICAD_DATA build-time variable to determine the default stock
library search path on Linux instead of relying on DEFAULT_INSTALL_PATH
in combination with hardcoded folder names. The search logic for Windows
and macOS is refactored but remains unchanged.

INSTALL.txt

@@ -55,8 +55,6 @@ Linux install tree
   ${prefix}/share/kicad/modules/packages3d - 3D component models (.wrl and .step format).
   ${prefix}/share/kicad/resources          - Resource files (images, etc).
 
-  KiCad searches for the libraries, templates, 3D models, etc. bin/../share.
-
   Warning:
   Do not change the KiCad tree, or the location of binary files.  Otherwise
   KiCad may not be able to find some or all of its required files.

common/pgm_base.cpp

@@ -281,19 +281,10 @@ bool PGM_BASE::InitPgm()
         return false;
 
     wxFileName baseSharePath;
-#if defined( __WXMSW__ )
-    // Make the paths relative to the executable dir as KiCad might be installed anywhere
-    // It follows the Windows installer paths scheme, where binaries are installed in
-    // PATH/bin and extra files in PATH/share/kicad
-    baseSharePath.AssignDir( m_bin_dir + "\\.." );
-    baseSharePath.Normalize();
+#ifdef __WXMAC__
+    baseSharePath.AssignDir( PATHS::GetOSXKicadMachineDataDir() );
 #else
-    baseSharePath.AssignDir( wxString( wxT( DEFAULT_INSTALL_PATH ) ) );
-#endif
-
-#if !defined( __WXMAC__ )
-    baseSharePath.AppendDir( "share" );
-    baseSharePath.AppendDir( "kicad" );
+    baseSharePath.AssignDir( PATHS::GetStockDataPath( false ) );
 #endif
 
     // KICAD6_FOOTPRINT_DIR

common/systemdirsappend.cpp

@@ -67,6 +67,12 @@ void SystemDirsAppend( SEARCH_STACK* aSearchStack )
     // Use as second ranked place.
     maybe.AddPaths( wxT( DEFAULT_INSTALL_PATH ) );
 
+#ifdef __WXGTK__
+    // On Linux, the stock data install path is defined by KICAD_DATA.
+    // Useful when multiple versions of KiCad are installed in parallel.
+    maybe.AddPaths( PATHS::GetStockDataPath( false ) );
+#endif
+
     // Add the directory for the user-dependent, program specific data files.
     // According to wxWidgets documentation:
     // Unix: ~/.appname