Though it is highly encouraged to do out-of-source builds, it is not
strictly forbidden to do in-source builds. So we should ignore the files
generated by CMake. Only tested with a Linux build, with both make and
ninja backends.
I added .dll and .dylib versions (for Windows and macOS respectively),
guessing these will be the file names on these platforms, unless
mistaken (since untested).
As discussed in !10, let's add with this commit files generated by the
build system, but not any personal environment files (specific to
contributors' environment).
If I missed any file name which can be generated by the build system in
some platforms, configuration, or condition, let's add them as we
discover them.
