CMake Nightly Date Stamp
Fix some generator crashes on undefined CMAKE_BUILD_TYPE

Since commit 84fdc9921 (stringapi: Pass configuration names as strings,
2014-02-09), it is not safe to use GetDefinition("CMAKE_BUILD_TYPE")
without checking the return value.  Use GetSafeDefinition instead so
that a missing definition is treated as an empty string like code paths
did prior to the above commit.

Reported-by: Richard Wirth <richard@califax.de>
Merge topic 'stringapi-crash-cleanup' into next

c491cb1e Fix some generator crashes on undefined CMAKE_BUILD_TYPE
562d5f57 CMake Nightly Date Stamp