Fix support for older CMake

[ferdnyc]

I used add_compile_definitions() in #515, which is CMake 3.12+ only. (Oops.) Switch to add_definitions() which has been supported since the dawn of time.

[codecov-commenter]

Codecov Report

Merging #517 into develop will increase coverage by 0.04%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #517      +/-   ##
===========================================
+ Coverage    48.23%   48.28%   +0.04%     
===========================================
  Files          128      128              
  Lines         9949    10132     +183     
===========================================
+ Hits          4799     4892      +93     
- Misses        5150     5240      +90     

Impacted Files	Coverage Δ
src/FFmpegReader.cpp	68.79% <0.00%> (+0.46%)	⬆️
src/Clip.cpp	41.25% <0.00%> (+1.49%)	⬆️
src/QtImageReader.cpp	61.32% <0.00%> (+4.71%)	⬆️

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[ferdnyc]

*sigh* NOPE. It seems I broke Travis again, somehow. 😠

[ferdnyc]

@jonoomph I don't know what happened here, but if you're able to either unfuck Travis or forcibly merge this, that'd be a help. I'm guessing at least one of the Gitlab builders is currently breaking on the CMake 3.12 command I accidentally merged into the repo (and now can't fix).

[ferdnyc]

@jonoomph This change, BTW, was made for the Debian folks, in hopes of getting a newer version of OpenShot finally packaged into their repos. See #515, OpenShot/libopenshot-audio#97, and most of all the Debian bug I linked to from those, for all the gory details.