This attempts to clean up the autoconf script a bit and follow the
upstream division of generic code in config/ with the actual lookup
configuration in configure.in. Also updated our installation to rely
on a more modern version of autoconf by backporting parts of upstream
commit 7cc514ac. This commit consist of:

  * Decouple --enable-codegen and --with-codegen-prefix to not
    silently ignore prefixes if the enable flag isn't passed.
	Emit a warning if configuring prefixes without codegen. Also
	moves --with-codegen-prefix to require an argument since
	--with-codegen-prefix without an argument is likely to hide
	either a scripting bug or a misunderstanding from the user
  * Move program checks for cmake and apr-1-config to programs.m4
    and allow for path overrides and ensure to use the resolved
	path when invoking cmake for --enable-codegen
  * Propagate the apr-1-config flags and objects to where used via
    Makefile.global rather than performing another lookup
  * Remove check for unused arguments since autoconf does that
    automatically since 2.63
  * Remove backported fseeko handling since that isn't relevant
    for modern autoconf versions
  * Minor help output tidying and spelling fixes