fix compiling VideoIO with JuliaC

[bjarthur]

here is a minimal working example for which compilation fails:

module FFMPEGTest

using VideoIO

(@main)(args) = return 0

end # module FFMPEGTest

]generate FFMPEGTest and put the above in src/FFMPEGTest.jl.

then ]app add JuliaC@0.2.3

finally ~/.julia/bin/juliac --output-exe foo --bundle build FFMPEGTest

all sort of errors result. this PR fixes it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.45%. Comparing base (c8f9b00) to head (22eb67a).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #452      +/-   ##
==========================================
- Coverage   80.05%   79.45%   -0.61%     
==========================================
  Files          10       10              
  Lines        1499     1499              
==========================================
- Hits         1200     1191       -9     
- Misses        299      308       +9     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[giordano]

Feels like these should be enums, not (non-const) globals

[giordano]

To be clear, enum is a thing in Julia: https://docs.julialang.org/en/v1/base/base/#Base.Enums.@enum

[bjarthur]

oh! yes. just changed it to conform to how clang handles enums elsewhere in this file. e.g. https://github.com/JuliaIO/VideoIO.jl/blob/master/lib/libffmpeg.jl#L143-L155