Add alternative build targets

[ChrisRackauckas]

Build outputs for C, Stan, and "Octave"

[codecov]

Codecov Report

Merging #257 into master will increase coverage by 0.61%.
The diff coverage is 95.71%.

@@            Coverage Diff             @@
##           master     #257      +/-   ##
==========================================
+ Coverage   86.33%   86.95%   +0.61%     
==========================================
  Files          15       16       +1     
  Lines         571      598      +27     
==========================================
+ Hits          493      520      +27     
  Misses         78       78

Impacted Files	Coverage Δ
src/utils.jl	62.29% <ø> (-12.71%)	⬇️
src/ModelingToolkit.jl	50% <ø> (ø)	⬆️
src/build_function.jl	95.71% <95.71%> (ø)

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