fix: link webkit2gtk and X11 on every Linux build, not only Flatpak (#15972)

libslic3r_gui calls webkit_* directly, and OrcaSlicer.cpp and libspnav
call Xlib, but both libraries were only linked when FLATPAK was set.
The default build links because the bundled static wxWidgets lists
them in wx-config. A shared wxWidgets does not, so any build against
one, like the Flatpak build or a distro package, fails with undefined
webkit_* and X* symbols.

src/slic3r/CMakeLists.txt

@@ -1019,14 +1019,9 @@ if (UNIX AND NOT APPLE)
     find_package(GTK${SLIC3R_GTK} REQUIRED)
     pkg_check_modules(LIBSECRET REQUIRED libsecret-1)
     pkg_check_modules(webkit2gtk REQUIRED webkit2gtk-4.1)
-    if (FLATPAK)
-        # I don't know why this is needed, but for whatever reason slic3r isn't
-        # linking to X11 and webkit2gtk. force it.
     find_package(X11 REQUIRED)
-        target_link_libraries(libslic3r_gui ${X11_LIBRARIES} ${webkit2gtk_LIBRARIES})
-    endif()
     target_include_directories(libslic3r_gui SYSTEM PRIVATE ${GTK${SLIC3R_GTK}_INCLUDE_DIRS} ${LIBSECRET_INCLUDE_DIRS} ${webkit2gtk_INCLUDE_DIRS})
-    target_link_libraries(libslic3r_gui ${GTK${SLIC3R_GTK}_LIBRARIES} fontconfig ${LIBSECRET_LIBRARIES})
+    target_link_libraries(libslic3r_gui ${GTK${SLIC3R_GTK}_LIBRARIES} fontconfig ${LIBSECRET_LIBRARIES} ${webkit2gtk_LIBRARIES} ${X11_LIBRARIES})
 
     # Propagate GDK backend detection results as compile definitions so that
     # LinuxDisplayBackend.cpp can include the right GDK headers.