Fix MSVC cross compilation

[Faless]

Currently, the DefaultCFlags.cmake overrides the
CMAKE_STATIC_LINKER_FLAGS to suppress linker warnings about files with no symbols defined.

This has the side effect of breaking MSVC cross compilation (where CMAKE_STATIC_LINKER_FLAGS is used to specify the /MACHINE:ARCH flag)

This commit make sure we append to CMAKE_STATIC_LINKER_FLAGS instead of replacing its values

The error when trying to cross compile with MSVC is the following:

[100%] Linking C static library ..\..\git2.lib
LINK : warning LNK4068: /MACHINE not specified; defaulting to X64
..\util\CMakeFiles\util.dir\alloc.c.obj : fatal error LNK1112: module machine type 'ARM64' conflicts with target machine type 'x64'

[ethomson]

Thanks for the fix!