`configure --enable-doc` is broken by design

[sprintersb]

configure --enable-doc only works when there is already a working toolchain including AVR-LibC installed. Otherwise:

$ make
...
Making all in api
make[3]: Entering directory '$builddir/doc/api'
if test -n "$builddir/avr/lib/avr4/exit.o $builddir/avr/devices/atmega8/crtatmega8.o -lgcc" ; then \
	NODEFLIBS="-nostartfiles -nodefaultlibs" ; \
fi; \
cd $srcdir/doc/examples/demo && \
	make DEFS="-I$srcdir/include"  \
		LIBS="$NODEFLIBS $builddir/avr/lib/avr4/exit.o $builddir/avr/devices/atmega8/crtatmega8.o -lgcc" all
make[4]: Entering directory '$srcdir/doc/examples/demo'
avr-gcc -g -Wall -O2 -mmcu=atmega8 -I$srcdir/include   -c -o demo.o demo.c
In file included from demo.c:16:
$srcdir/include/avr/io.h:729:10: fatal error: avr/version.h: No such file or directory
  729 | #include <avr/version.h>
      |          ^~~~~~~~~~~~~~~
compilation terminated.
make[4]: *** [<builtin>: demo.o] Error 1
make[4]: Leaving directory '$srcdir/doc/examples/demo'
make[3]: *** [Makefile:1209: demo] Error 2
make[3]: Leaving directory '$builddir/doc/api'
...

avr/version.h is not present in $srcdir of course, and is only available in $prefix after make install.

Possible work-arounds:

1. 1. configure and install without --enable-doc
   2. In $builddir/doc/api: make html ...; make install-dox-html ...
      When you want to install all flavours (HTML, PDF, man, ...) then just do make install.
      (Notice that the example sources have already been installed by the vanilla install.)
2. 1. First install a complete toolchain by whatever means.
   2. configure with --enable-doc etc.

Notice that A. is faster than configuring and building AVR-LibC twice.