Fix a few build issues.

Here is a patch for a few outstanding issues.  Some have been mention on the NWNX forum, some not.

best,
jmd

Changes:
- .gitignore: Add lines for autoconf generated files.
- CMakeLists.txt: Set cmake policy CMP0018 to OLD for
  compatibility with cmake 2.8.9 and higher.  See:
  http://www.manpagez.com/man/1/cmakepolicies/
- configure: Delete, doesn't need to be commited.
- install.sh: Script was adding 'plugins/$plugins' to
  extraplugins string which when passed to configure
  would then also add 'plugins/' and would attempt to find
  'plugins/plugins/$plugin'.
- plugins/optimizations: Rename Makefile -> Makefile.in.  Not sure the intention, but
  cmake clobbers it.
- plugins/spells: Add -Wno-error=unused-but-set-variable.