Otherwise, each source file that includes the header will create a
new definition, which are usually merged together by the linker.
Multiple definitions of an object is not allowed in ISO C.
This rationalises the path construction and basename file
operations. The default implementation is POSIX which works for all
frontends except windows, riscos and amiga which have differeing path
separators and rules.
These implementations are significantly more robust than the previous
nine implementations and also do not use unsafe strncpy or buffers
with arbitrary length limits.
These implementations also carry full documentation comments.
