Commit v3.4.0-rc1~494^2~4 (cmMakefile: Add API for elseif to create
backtrace., 2015-05-29) removed the use of cmMakefileCall to push/pop
execution context in favor of a new way to create backtraces.

However, a call to cmMakefile::GetExecutionContext is still invoked to
issue a contextual CMP0054 warning through cmConditionEvaluator.  As
the elseif is not part of the call stack, this resulted in trying to
access an empty vector.

Avoid the attempt at getting execution context when evaluating elseif by
constructing a context and backtrace on behalf of the cmConditionEvaluator
in all cases.