Makefile.vc: remove deprecated /Gm option
Makefile.vc: remove deprecated /Gm option

this was only used in debug builds, the build should be fast enough in
either case.

https://docs.microsoft.com/en-us/cpp/build/reference/gm-enable-minimal-rebuild?view=vs-2019
/Gm is deprecated. It may not trigger a build for certain kinds of
header file changes. You may safely remove this option from your
projects. To improve build times, we recommend you use precompiled
headers and incremental and parallel build options instead.

Change-Id: I8e3c0e7cc82e10bd1d2b0904d290fe4e050ebe8b

webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp