Configure does not handle well the case of doxygen not found, but --enable-docs is used

[edwardhartnett]

If --enable-doxygen is used with configure, but doxygen is not found, instead of erroring out in configure, the build gives this warning:

checking for doxygen... no
configure: WARNING: Doxygen not found - documentation will not be built

And then fails in the docs directory with this confusing message:

make[3]: Entering directory '/home/runner/work/netcdf-c/netcdf-c/docs'
Doxyfile \
cp auth.md obsolete/fan_utils.html html
/bin/bash: line 1: Doxyfile: command not found
make[3]: *** [Makefile:825: doxyfile.stamp] Error 127

Instead, configure should error out and tell the user to install doxygen, or build without --enable-doxygen.

I will fix.