build-infra: backslashes at end of LIB and INCLUDE in spec.gmk

Sometimes vcvars*.bat seems to generate LIB and INCLUDE variables with a backslash at the end. This breaks make in the generated spec.gmk since the backslash cancels out the next line. We either need to filter away any trailing backslash or add newlines between these export lines in spec.gmk.in.

Reported by Phil Race on the build-infra-dev.