Don't build all object files with -ZW, only the one that needs it

This reduces the build time from 69 s to 30 s, reduces the size of
the built wels.lib from 30 MB to 3.9 MB, and reduces the number of
warnings when building wels.lib.
Merge pull request #766 from mstorsjo/reduce-msvc-runtime-flags

Don't build all object files with -ZW, only the one that needs it