Autotools detect YAML library
I only have some comments about the shell scripting elements.



This is a very archaic (and silly) way of testing flags:


    [test "x$enable_yaml" != "xno"],



You can write simpler as:

    [test "$enable_yaml" != no],




This can be simplified:


    if test "${with_yamlroot}" != ""; then



To just:

    if test "${with_yamlroot}"; then


I also find it odd to use sometimes $this_style and sometimes ${that_style}.
I prefer less typing (especially the shift key for }),
so the first, simpler style.
The only time I use braces is when appending some text directly after the variable would break the script, for example in situations like this:

authtype=...  # something
# broken, not using the $authtype variable
#modpath=/usr/libexec/apache2/mod_$authtype_dbd.so
# good
modpath=/usr/libexec/apache2/mod_${authtype}_dbd.so