cmCPackGenerator: Avoid crash on undefined CMAKE_MAKE_PROGRAM

Since commit 6557382d (stringapi: Use strings for program
paths, 2014-02-24), cmGlobalGenerator::GenerateBuildCommand
takes the make program path as a std::string.  Fix the lookup
in cmCPackGenerator::InstallProjectViaInstallCMakeProjects to
use GetSafeDefinition for CMAKE_MAKE_PROGRAM to avoid possible
construction of std::string from a NULL.

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

3bae343f cmCPackGenerator: Avoid crash on undefined CMAKE_MAKE_PROGRAM