cmake: list_spaces_append_once fails with spaces in filename

Windows standard libraries are located in C:/Program Files/Microsoft
SDKs/[...]. They are already included in the default MSVC
LIBPATH. Hence, find_library(WSOCK32_LIBRARY wsock32) and
find_library(WS2_32_LIBRARY ws2_32) are not needed. They return the full
path to the libraries including spaces. Of course,
list_spaces_append_once will mangle the result and the build fails.

Bug: http://curl.haxx.se/bug/view.cgi?id=3494968