Use macros for setting the output directory

This makes sure this is set to the exact same string in all
the configurations, simplifying editing multiple configurations
at the same time.

This changes the output directory for 64 bit binaries from
bin/win64 to bin/x64, but this is the common pattern used by
MSVC in new projects.
Merge pull request #1194 from mstorsjo/msvc-simplify-output-dirs

Use macros for setting the output directory in MSVC project files