MSVC release builds: Ensure that /LTCG is used

MSVC release builds: Ensure that /LTCG is used

MSVC 2015 changed its default link-time code generation setting to
/LTCG:incremental, which causes problems if /opt:noref is to be used,
meaning that some code will be optimized out by the linker.

Avoid this situtation here by enforcing the use of /LTCG for MSVC 2010+
builds.