Cleanup PackageInfo.g
PackageInfo.g: don't set 'Autoload' (which only GAP <= 4.4 uses)
Add .mailmap
PackageInfo.g: update WWW and download URLs
build: add autogen.sh
build: remove generated files
Update .gitignore
build: remove targets 'archive', 'towww'
build: avoid installing stuff into PWD
build: fix AM_SILENT_RULES, some other tweaks

In particular, require automake >= 1.11; enable automake
warnings; and fix one of those warnings (we need to use
AM_PROG_AR, and that has to be called before LT_INIT)
makedoc.g: regenerate VERSION file
build: cleanup
build: make AC_INIT use VERSION file & github links
Revert SIGCHLD handler changes

Revert commits 3905afa9 and 48f0f1fd for now, see issue #5.
build: update 3rd party m4 files to latest versions
PackageInfo.g: cleanup
Update CHANGES
Remove misleading ref to 5.0 release from HISTORY
Change version to 4.3

The last released version was 4.2. There almost was a 5.0, with
SIGCHLD handler changes in it, but it never saw the light of
day. Since we don't have major changes so far, the next
version should tentatively be called "4.3".