test_vars: also check @IS_MSVC@ content
test_vars: also check @IS_MSVC@ content

On autotools if @IS_MSVC@ is not defined, it is left as a string
literal, instead of being nothing. Because of that, the content
is now also checked. For CMake and MSVC, @IS_MSVC@ will be '1'.
Otherwise, it can be anything else.

tarruda/check,tarruda/check,tarruda/check,tarruda/check,tarruda/check