allow to override libdir and includedir

extra check to silence the static analyzer

fixed a race condition introduced in 8615946

and shipped in 2.8 release

minor cleanings

provide a FBUF_STATIC_INITIALIZER_POINTER() macro

to simplify (and speedup) initialization of a static fbuf structure.

This is handy when you have a preallocated structure which contains an fbuf_t member
(not a pointer) and we want to set custom parameters without having to call multiple functions to do that
(and obviously FBUF_STATIC_INITIALIZER can't be used because we are not in a declaration statement)

ensure returning 0 on success

the ht_call callback can return 1 to commence removal of the processed
item, but ht_call() itself should return 0 because it succeeded

fixed the documentation for the fbuf_detach() function

if the table is being grown we need to wait for it to finish

if a thread is accessing a list from the previous items array and later locking the list
is not safe because there is a race and the list (or the old items array) might be released
by the thread growing the table before the list is being locked by the initial thread

better concurrency test

fixed the algorithm to cope with some corner cases triggered by stres…

…s tests

the library now looks really safe and has been tested under heavy concurrency load