Move configure.in to configure.ac per autoconf guidelines
Use AM_CPPFLAGS instead of INCLUDES per automake guidelines
make: Use vala-0.12 as newer versions are not backwards-compatible
Fix several ambiguities in vala files
Remove trailing whitespace
Fix out-of-source builds
doc: Make documentation optional
Update INSTALL file
make: Tell autotools to use POSIX tar

Otherwise, distributable packages can't be made due to too long
filenames.