build: more fine-grained dependencies for documentation build tools

Perl is needed to build the manual pages, texi2html for the HTML pages.
ac3dec: Fix out-of-array read

Signed-off-by: Diego Biurrun <diego@biurrun.de>
mpegaudiodec: Consistently handle fixed/float templating
Add missing #includes for *INT64_MAX and *INT64_C