OK, now I'm really done monkeying with the compiler-flags macros.

* configure.ac: Rename the following command-line switches:
   --enable-gcc-warnings             -> --enable-extra-warnings
   --enable-gcc-warnings-are-errors  -> --enable-warnings-are-errors
   --enable-gcc-hardening            -> --enable-hardening
  More shell logic pulled out to .m4 files...
* 00backports.m4: add AS_VERSION_COMPARE.
* libevent_cflags.m4: fix for clang.
* libevent_cflags_sets.m4: new file (contents pulled out of configure).
* libevent_gc_sections.m4: rename libevent_function_sections.m4; macro
  renamed LIBEVENT_FUNCTION_SECTIONS; pull the command-line conditional
  inside the macro.

* libevent_openssl.m4: Add an AC_MSG_CHECKING/RESULT pair.