add Dbxref and Ontology_term syntax checking
src_clean
fix case
allow compiling with assert=no
allow compiling with amalgamation=yes
adjust XRF checker behaviour to be similar to type checker
initialise variable differently

This fixes a scan-build warning.
Merge pull request #167 from satta/xrf

Implement XRF_abbr parsing and Dbxref/Ontology_term checking
make -threads a developer option
remove file name from error messages

While a GtBioseq can have a file name, this is not the
case for any implementation of the GtSeqCol. This commit
removes file names from the respective error messages
to avoid getting "(null)" as a file name.
update SQLite to 3.8.2
remove SQLite from the amalgamation