Fix CMakeLists.txt for windows

FILE(MAKE_DIRECTORY ${_tgtdir}/${_subdir}) throws error on ${CMAKE_CURRENT_BINARY_DIR}/webodf/webodfversion.js
As ${_subdir} is an absolute path like /E:/Projects/build/webodf and windows doesn't like concatenate two abolute paths unfortunately
Handles platform-specific file separator in updateJS.js

Currently broken on windows as it uses backslash instead of slash for file separator
Merge pull request #257 from kevinlli/buildfix

Address a couple of build script issues (windows related)