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.
Only use the -Gm option in desktop debug MSVC builds

This option conflicts with -ZW which is used in Windows Phone builds.
Merge pull request #766 from mstorsjo/reduce-msvc-runtime-flags

Don't build all object files with -ZW, only the one that needs it
Merge pull request #767 from mstorsjo/avoid-conflicting-msvc-options

Only use the -Gm option in desktop debug MSVC builds