Use C99 inline in more places
Move some inline methods to their appropriate headers
Forward some wrapper functions to the base method
Remove the Lock and Unlock methods from BackendFuncs

All backends that still use the old interface use the default locking methods,
which is also used by the ALCbackend base.
Include strings.h when available
Set -D_GNU_SOURCE=1 earlier
Explicitly define _POSIX_C_SOURCE and _XOPEN_SOURCE

Set them to 200809L and 700 respectively, instead of assuming _GNU_SOURCE will
do it.