Drone: use generate function to build jobs

[sdarwin]

Alan has abstracted the standard cpp.al drone jobs into a generate function that can be used on all repos, so adding a new compiler only needs to be done once in a central location.

generate will probably be moved to the boost-ci repository eventually.

Specific notes about json: gcc 4.8 and 4.9 were using Ubuntu 14.04 Trusty before. When switching those to Ubuntu 16.04 Xenial there are errors. Skipping 4.8 and 4.9 now. You could test locally, or change .drone.star to 'gcc >=4.8' instead of 'gcc >=5.0'. Since Ubuntu 14.04 is nearly obsolete, that operating system should not continue to be used.

[codecov[bot]]

Codecov Report

Merging #836 (48b75c7) into develop (607b21d) will decrease coverage by 0.04%. The diff coverage is n/a.

:exclamation: Current head 48b75c7 differs from pull request most recent head 7d1fe09. Consider uploading reports for the commit 7d1fe09 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #836      +/-   ##
===========================================
- Coverage    99.00%   98.95%   -0.05%     
===========================================
  Files           70       71       +1     
  Lines         6837     7088     +251     
===========================================
+ Hits          6769     7014     +245     
- Misses          68       74       +6     

Impacted Files	Coverage Δ
include/boost/json/value_from.hpp	87.50% <0.00%> (-12.50%)	:arrow_down:
...nclude/boost/json/detail/impl/default_resource.ipp	87.50% <0.00%> (-12.50%)	:arrow_down:
include/boost/json/impl/parse.ipp	96.07% <0.00%> (-3.93%)	:arrow_down:
include/boost/json/impl/value_ref.ipp	96.00% <0.00%> (-1.30%)	:arrow_down:
include/boost/json/detail/value_to.hpp	98.87% <0.00%> (-1.13%)	:arrow_down:
include/boost/json/impl/object.ipp	99.76% <0.00%> (-0.24%)	:arrow_down:
include/boost/json/array.hpp	100.00% <0.00%> (ø)
include/boost/json/parser.hpp	100.00% <0.00%> (ø)
include/boost/json/string.hpp	100.00% <0.00%> (ø)
include/boost/json/value_ref.hpp	100.00% <0.00%> (ø)
... and 33 more

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[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/836/pullrequest.html

[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/836/pullrequest.html