Remove unused includes.
Use external libjemalloc in preference to the in-tree copy, which
is removed. mallinfo() is implemented with the new mallctl().
Reflect jemalloc's status as a dependency in configure output and
in packaging.

Co-authored-by: Gabriele Giacone <1o5g4r8o@gmail.com>
Co-authored-by: Bastiaan Jacques <bastiaan@bjacques.org>
Remove various configure-time checks for jemalloc.

Distributions can be expected not to have a package for jemalloc if
their platform is too esoteric for jemalloc to work (upstream
libjemalloc now supports all major platforms). Users building on such
platforms can be expected to disable jemalloc support.
Trivial cleanups.
Fix warnings.
Use references to avoid possible copying.
Const.
Avoid NULL dereference and assorted cleanups.
Prefix instead of postfix.
Don't bother modifying a local variable that's not returned.
Avoid reading past the end of codeMap.
empty().
Fix check for install deps script.
doc/C: fix info doc generation.
doc/C: remove multiple makeinfo execution.
Fix warnings: variables set but not used.
doc/C: fix line wrap.
Suppress agg headers warnings.
Fix warnings: cast discards '__attribute__((const))' qualifier.
Fix warnings: -Wdeprecated-declarations.