Fix linking with libpthread

WebKit use libpthread directly but is depending on other qt modules
causing it to be linked against, which might break unless -lpthread
is last. Instead just add it explicitly after the static libraries.

Change-Id: I2b95cff2c96373f8dce6f95052c4fccbe1982b33
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>

Tools/qmake/mkspecs/features/default_post.prf

@@ -201,6 +201,7 @@ needToLink() {
         linkAgainstLibrary($$library, $$eval(WEBKIT.$${library_identifier}.root_source_dir))
         LIBS += $$eval(WEBKIT.$${library_identifier}.dependent_libs)
     }
+    posix:!darwin: LIBS += -lpthread
 }
 
 creating_module {