expand Boost_LIBRARIES for pkg-config libs

[jrmarkle]

With the latest cmake build process the installed file libvw.pc contains all the required boost libraries in a semicolon-separated list. This causes the output of pkg-config --libs libvw on Ubuntu 18.04 to be:

-L/usr/local/lib -lvw -lallreduce -pthread /usr/lib/x86_64-linux-gnu/libboost_program_options.so\;/usr/lib/x86_64-linux-gnu/libboost_system.so\;/usr/lib/x86_64-linux-gnu/libboost_thread.so\;/usr/lib/x86_64-linux-gnu/libboost_unit_test_framework.so\;/usr/lib/x86_64-linux-gnu/libboost_chrono.so\;/usr/lib/x86_64-linux-gnu/libboost_date_time.so\;/usr/lib/x86_64-linux-gnu/libboost_atomic.so\;/usr/lib/x86_64-linux-gnu/libpthread.so -lz

which is not useful for passing to the linker.

This change separates the library paths with spaces so that a command such as g++ $(pkg-config --libs libvw) ... can work.

There might be a better, more idiomatic cmake way to fix this.

[jackgerrits]

Thanks for this fix @jrmarkle. I'm not aware of a more idiomatic way to do this, so looks good.

[JohnLangford]

Merged, thanks :-)