Actived alternative threaded implementation.
Compile only one of the two threaded versions.
Excluded more irrelevant code for partition_threading.
Completed stream based threading of when sorting buckets.
Started to synchronize the results computed by the threads.
Buckets sorted by threads are finally synchronized.
Moved mutexes into the structure they protect.
Satisfy compile and src_check
silence warnings for pkg-config
Merge pull request #235 from stefan-kurtz/sk

Add some code related to threading of suffixerator; cosmetics
Merge pull request #236 from satta/silence_warnings

silence warnings for pkg-config
update docs
Merge pull request #238 from Garonenur/documentation

update docs
improve xdrop

add test for backward xdrop for perfect matches
adjust comment
fix positions

xdrop positions in ltrharvest_stream corrected
remove zero length xdrop

do not call xdrop with zero length

add assertion to GtSeqabstract to prevent zero length sequences
positions and assertions

there is the possibility for GtSeqabstracts to have zero length now.
I fixed every occurence where they are used with xdrop, but some code
uses GtSeqabstracts not with xdrop. There zero length GtSeqabstracts
happen.
Merge pull request #240 from Garonenur/GtSeqabstract

Gt seqabstract
fix node order for sequence/EOF nodes
fix failing Ruby tests on 64-bit Linux