configure.ac: export an empty lib_prefix variable
configure.ac: export an empty lib_prefix variable

this fixes the pkg-config files after:
2498209b *.pc.in: add lib prefix to lib names w/MSVC

Change-Id: I1c932103da175ca50826a8801796f0f6ee5ef583

webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp