Do not try to build QtWebKit as a static library

QtWebKit is too large to be linked as a static library on most platforms,
since static libraries are typically limited to 4Gbyte even on 64bit
tool chains (http://sourceware.org/bugzilla/show_bug.cgi?id=14625).
The GNU ar thin archives do not have that limitation however.

Task-number: QTBUG-32309
Change-Id: Ia290ecb6f54fc0a0482f17c6d9dc2ad0c810b3b3
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>
Do not completely erase LIBS variable

Using an empty LIBS variable prevents usage of custom library paths
and libraries via configure -L <path> -l <extra_lib>.
This is needed for linking with an alternative stdlib implementation.

Change-Id: Ie1009bfd435436bf584b2963066535ee90f4d5d0
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@digia.com>