Postgis building with `make docs`, “cannot parse /html/docbook.xsl”

When I try to build the docs, I'm getting,

make -C doc html
make[1]: Entering directory '/home/ecarroll/code/postgis/doc'
make -C html/image_src images
make[2]: Entering directory '/home/ecarroll/code/postgis/doc/html/image_src'
make[2]: Nothing to be done for 'images'.
make[2]: Leaving directory '/home/ecarroll/code/postgis/doc/html/image_src'
/usr/bin/xsltproc --param section.autolabel 1 --param section.label.includes.component.label 1 --param chunk.section.depth 0 --param generate.section.toc.level 2 --param funcsynopsis.style kr --param admon.graphics 1 --param admon.textlabel 0 --param simplesect.in.toc 0 --param use.id.as.filename 1 --param chunk.quietly 1 --nonet --stringparam html.stylesheet style.css  \
    --output html/postgis.html \
    /html/docbook.xsl \
    postgis-out.xml
warning: failed to load external entity "/html/docbook.xsl"
cannot parse /html/docbook.xsl
Makefile:258: recipe for target 'html/postgis.html' failed
make[1]: *** [html/postgis.html] Error 4
make[1]: Leaving directory '/home/ecarroll/code/postgis/doc'
GNUmakefile:89: recipe for target 'docs' failed
make: *** [docs] Error 2