Only enable our CUDA extensions when the user explicitly asks for them

[mborland]

Closes: #1383

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.35%. Comparing base (cfe946f) to head (1d9e927).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1384   +/-   ##
========================================
  Coverage    95.35%   95.35%           
========================================
  Files          825      825           
  Lines        68203    68203           
========================================
+ Hits         65033    65034    +1     
+ Misses        3170     3169    -1     

Files with missing lines	Coverage Δ
include/boost/math/special_functions/lanczos.hpp	100.00% <ø> (ø)
include/boost/math/special_functions/next.hpp	95.74% <ø> (ø)
include/boost/math/special_functions/sign.hpp	100.00% <ø> (ø)

... and 1 file with indirect coverage changes

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