Set -Zi instead of -ZI when building with MSVC in debug mode

The -ZI option is only supported when building for 32 bit x86,
when building for x64 or for arm, it gives the warning "/ZI is
not supported on this platform; enabling /Zi instead".

-ZI is for "edit & continue" debugging, which one probably wouldn't
be doing with a build outside of the IDE anyway, so keep it simple
instead of trying to set -ZI specifically for x86 32 bit builds.
Merge pull request #1135 from mstorsjo/msvc-make-debug-option

Set -Zi instead of -ZI when building with MSVC in debug mode