Use Refs rather than globals for atsign-requires dependencies

[timholy]

This appears to fix problems involving Compose, Cairo, and Fontconfig. The idea is to avoid using variables that get defined only via global statements that run only in @require blocks, and switch to a strategy to uses variables that are defined at precompilation time. Here I do that by defining the variables as Refs, so I can update their contents when they get initialized.

Fixes GiovineItalia/Gadfly.jl#1206
Replaces #341

This is not based on deep knowledge of what packages are or are not needed for my plotting task. I was simply following https://github.com/JuliaGraphs/GraphPlot.jl and then responded to messages about needing to import Cairo, Fontconfig.

[codecov-io]

Codecov Report

Merging #342 into master will increase coverage by 0.03%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #342      +/-   ##
==========================================
+ Coverage   38.05%   38.09%   +0.03%     
==========================================
  Files          18       18              
  Lines        3127     3124       -3     
==========================================
  Hits         1190     1190              
+ Misses       1937     1934       -3

Impacted Files	Coverage Δ
src/pango.jl	0% <0%> (ø)	⬆️
src/Compose.jl	20.23% <0%> (+0.69%)	⬆️

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[bjarthur]

thanks tim!