buildsys: teach package boostrap to use curl if available

[fingolfin]

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

Resolves #1866

[coveralls]

Coverage remained the same at 85.256% when pulling 74f288f on fingolfin:mh/download into ce16a7d on gap-system:master.

[sebasguts]

looks good to me :)

[codecov]

Codecov Report

Merging #3504 into master will increase coverage by 0.68%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3504      +/-   ##
==========================================
+ Coverage   84.75%   85.43%   +0.68%     
==========================================
  Files         698      699       +1     
  Lines      346577   346766     +189     
==========================================
+ Hits       293728   296252    +2524     
+ Misses      52849    50514    -2335

Impacted Files	Coverage Δ
src/gasman.c	86.28% <0%> (-3.66%)	⬇️
src/gap.c	81.18% <0%> (-0.37%)	⬇️
lib/debug.g	23.57% <0%> (ø)
lib/algrep.gi	80.6% <0%> (+0.04%)	⬆️
lib/cyclotom.gi	80.9% <0%> (+0.08%)	⬆️
lib/ratfun.gi	91.19% <0%> (+0.08%)	⬆️
src/plist.c	95.06% <0%> (+0.08%)	⬆️
lib/meataxe.gi	87.54% <0%> (+0.08%)	⬆️
lib/oprtperm.gi	93.75% <0%> (+0.1%)	⬆️
lib/orders.gi	77.01% <0%> (+0.12%)	⬆️
... and 117 more