qt: skip makefile variables in LIBS

The only known variable is $(SUBLIBS) in LIBS which is a variable that doesn't exist in the
makefile. It's only found when Qt is compiled as dynamic libraries.
We just skip the variable.

Without this $(SUBLIBS) is passed to the Makefile with the other libs.
This is fine with autotools which ignores the undefined variable.
But meson doesn't like it.

Ref. #29585

modules/gui/qt/scripts/static_dirs.py

@@ -105,6 +105,8 @@ if __name__ == "__main__":
                 for lib in l:
                     if lib == '':
                         continue
+                    if lib == '$(SUBLIBS)':
+                        continue
                     if lib.startswith('-l'):
                         result += ' ' + lib
                     elif lib.endswith('.so') or lib.endswith('.a'):