TableGen.cmake: Use DEPFILE for Ninja Generator with CMake>=3.7.

CMake emits build targets as relative paths (from build.ninja) but Ninja doesn't identify absolute path (in *.d) as relative path (in build.ninja).
So, let file names, in the command line, relative from ${CMAKE_BINARY_DIR}, where build.ninja is.

Note that tblgen is executed on ${CMAKE_BINARY_DIR} as working directory.

Differential Revision: https://reviews.llvm.org/D33707

llvm-svn: 305961