Build: Fix install targets, add SLIC3R_FHS, check resource dir on GUI_App init

Fix #1515

src/slic3r.cpp

@@ -22,6 +22,8 @@
 #include <boost/nowide/cenv.hpp>
 #include <boost/nowide/iostream.hpp>
 
+#include "unix/fhs.hpp"  // Generated by CMake from ../platform/unix/fhs.hpp.in
+
 #include "libslic3r/libslic3r.h"
 #include "libslic3r/Config.hpp"
 #include "libslic3r/Geometry.hpp"
@@ -77,6 +79,10 @@ int main(int argc, char **argv)
     // The resources are packed to 'resources'
     // Path from Slic3r binary to resources:
     boost::filesystem::path path_resources = path_to_binary.parent_path() / "resources";
+#elif defined SLIC3R_FHS
+    // The application is packaged according to the Linux Filesystem Hierarchy Standard
+    // Resources are set to the 'Architecture-independent (shared) data', typically /usr/share or /usr/local/share
+    boost::filesystem::path path_resources = SLIC3R_FHS_RESOURCES;
 #else
     // The application is packed in the .tar.bz archive (or in AppImage) as 'bin/slic3r',
     // The resources are packed to 'resources'