This makes sure that 'make dist' works as intended and contains all
the required files.

This avoids defining _GNU_SOURCE unconditionally, and instead use
whichever system extensions apply to the current host.

The config.h file is too easy to misuse, and since the checks are now
very limited, there is no reason not to just use CFLAGS to pass the
few definitions that are actually used.

This allows to remove the 0-sized and autogenerated top-level
"documentation" files that are required in the default GNU flavour.

This avoids entering a subdirectory as first thing, and is better
practice with autotools.

The mode parameter is not optional when O_CREAT is passed, so pass a
default of 0644 to it.

Remove checks for programs that are not used, remove checks for
library headers that are not conditional, and for types, and for
functions that are not replaced.

will be conditonal switch for loading drcs data