fixed configuration for cre2 inclusion/exclusion
review of configuration script
fixed error preventing build when --disable-linux is used at configuration time
moved implementation of function environ from (vicare posix) to (vicare)

This allows using srfi 98 even when (vicare posix) is disabled at configure
time.
changed test file to be run even when (vicare posix) is not available
fixed checking for function in configure script in the wrong section
review of configuration script
changed srfi 45 to allow multiple values
added infrastructure for optional inclusion of srfi libraries
review of configuration script
reorganised hierarchy of internal libraries