Teach "make bootstrap" to use `curl` if `wget` is not available

[fingolfin]

On macOS, curl is available but wget is not.

[fingolfin]

Would be great to get this into 4.9. Shouldn't be too hard either. E.g. we could add an etc/download.sh shell script, which looks something like this (untested):

#/bin/sh
set -e
command -v wget >/dev/null 2>&1 && exec wget "$@"
command -v curl >/dev/null 2>&1 && exec curl -O "$@"
echo "Error, failed to download: neither wget nor curl available"
exit 1

Then change Makefile.rules to use that script.

Caveat: This disables a trick we use to make our Travis builds a bit more robust, by passing lots of extra options to wget in etc/ci-prepare.sh, like this:

make bootstrap-pkg-full WGET="wget -N --no-check-certificate --tries=5 --waitretry=5 --retry-connrefused"

It should still be possible to get this "robust" variant. Easiest would be to just add those options into download.sh (and similar options for curl), but I am not sure we really want that for every user. E.g. the --no-check-certificate worries me (and I am not sure why I added that in ae866b6 anyway, perhaps we should just drop it?)

Another option would be to simply replace WGET = wget -N in Makefile.rules by DOWNLOAD = etc/download.sh, and then use $(DOWNLOAD) instead of $(WGET) later on. This, it still is possible to insert a custom download command (so any PR doing all of this should of course also adjust etc/ci-prepare.sh accordingly).

[fingolfin]

As @alex-konovalov correctly pointed out to me, end users won't care about these targets, so no need to get this into 4.9 (won't hurt either, of course).

[olexandr-konovalov]

Put this under GAP 4.10 milestone to help @cdwensley