Fixing a load of issues related to autoconf and some small source-level issues to re-add clang support.
autoconf feature detection probably needs some addressing - partially as -Werror is applied late.
Fuzzy bsearch is useful for finding an entry in a prefix sum array
(value is >= ele[0], < ele[1]), and the reentrant version is good when
data needs to be passed to the compare function. Adapted from the code
used in pr_resolve.
Most of the guts of configure.ac have been moved to config.d and are then
brought in by m4_include. This will make maintaining configure.ac much easier.
Also drop use of PROGRAM and VERSION, using PACKAGE_NAME, PACKAGE_VERSION, and
on occasion, PACKAGE_STRING instead, and clean out some old files we no longer
need.