.gitignore: Ignore more generated files

The build process generates doc/libffi.info and fficonfig.h.in, so add
them to .gitignore.
Change double quotes in Makefile.am to single quotes.

This was originally done in PR #84, except the change was made to Makefile.in instead of Makefile.am and was therefore reverted the next time the files were regenerated.
Merge pull request #100 from rvandermeulen/bug-756740

Change double quotes in Makefile.am to single quotes.
Merge pull request #99 from joshtriplett/gitignore

.gitignore: Ignore more generated files