buildsys: teach package boostrap to use curl if available

This allows people to do a package bootstrap on macOS without installing wget,
as curl is installed there by default but wget is not.

Makefile.rules

@@ -1105,13 +1105,13 @@ PKG_BRANCH = master
 PKG_BOOTSTRAP_URL = https://www.gap-system.org/pub/gap/gap4pkgs/
 PKG_MINIMAL = packages-required-$(PKG_BRANCH).tar.gz
 PKG_FULL = packages-$(PKG_BRANCH).tar.gz
-WGET = wget -N
+DOWNLOAD = $(abs_srcdir)/etc/download.sh
 
 bootstrap-pkg-minimal:
 	@if test -e pkg; then \
         echo "The pkg directory already exists. Please move or remove it to proceed."; \
     else \
-        $(WGET) $(PKG_BOOTSTRAP_URL)$(PKG_MINIMAL) && \
+        $(DOWNLOAD) $(PKG_BOOTSTRAP_URL)$(PKG_MINIMAL) && \
         mkdir pkg && \
         cd pkg && \
         tar xzf ../$(PKG_MINIMAL) ; \
@@ -1121,7 +1121,7 @@ bootstrap-pkg-full:
 	@if test -e pkg; then \
         echo "The pkg directory already exists. Please move or remove it to proceed" ; \
     else \
-        $(WGET) $(PKG_BOOTSTRAP_URL)$(PKG_FULL) && \
+        $(DOWNLOAD) $(PKG_BOOTSTRAP_URL)$(PKG_FULL) && \
         mkdir pkg && \
         cd pkg && \
         tar xzf ../$(PKG_FULL) ; \

etc/ci-prepare.sh

@@ -46,15 +46,15 @@ time make V=1 -j4
 
 # download packages; instruct wget to retry several times if the
 # connection is refused, to work around intermittent failures
-WGET="wget -N --no-check-certificate --tries=5 --waitretry=5 --retry-connrefused"
+WGET="wget --tries=5 --waitretry=5 --retry-connrefused"
 if [[ $(uname) == Darwin ]]
 then
     # Travis OSX builders seem to have very small download bandwidth,
     # so as a workaround, we only test the minimal set of packages there.
     # On the upside, it's good to test that, too!
-    make bootstrap-pkg-minimal WGET="$WGET"
+    make bootstrap-pkg-minimal DOWNLOAD="$WGET"
 else
-    make bootstrap-pkg-full WGET="$WGET"
+    make bootstrap-pkg-full DOWNLOAD="$WGET"
 fi
 
 # check that GAP is at least able to start

etc/download.sh

@@ -0,0 +1,16 @@
+#/bin/sh
+#
+# This script expects a single argument, which should be an URL pointing to a
+# file for download; it then tries to download that file, into a local file
+# with the same name as the remote file.
+set -e
+
+# check whether curl is available, and if so, delegate to it
+command -v curl >/dev/null 2>&1 && exec curl -O "$@"
+
+# check whether wget is available, and if so, delegate to it
+command -v wget >/dev/null 2>&1 && exec wget -N "$@"
+
+# if no supported download tool is available, abort
+echo "Error, failed to download: neither wget nor curl available"
+exit 1