Add PREFER_SYSTEM_LIBRARIES support for 'v8' engine

[dwrobel]

It also fixes compilation errors when SUPPORT_V8 is defined.

Recommended options for compiling with 'v8' JS engine:
-DPREFER_SYSTEM_LIBRARIES=ON
-DSUPPORT_V8=ON
-DSUPPORT_NODE=OFF
-DSUPPORT_DUKTAPE=OFF
-DPKG_CONFIG_DISABLE_NODE=ON
-DPKG_CONFIG_DISABLE_NODE8=ON

For more comprehensive example please have a look at:
ci/builds/*-wayland-v8-gcc-tsan.sh script.

[dwrobel]

Please check the places I had to add std::ignore to compile it cleanly to make sure that return value can be safely ignored.

[codecov]

Codecov Report

Merging #1476 into master will decrease coverage by 0.98%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1476      +/-   ##
==========================================
- Coverage   78.19%   77.21%   -0.99%     
==========================================
  Files         114      123       +9     
  Lines       15037    17132    +2095     
==========================================
+ Hits        11758    13228    +1470     
- Misses       3279     3904     +625

Impacted Files	Coverage Δ
src/rtScriptV8/rtFunctionWrapper.h	56.52% <0%> (-10.15%)	⬇️
src/mac/pxEventLoopNative.mm	77.06% <0%> (-8.26%)	⬇️
src/rtThreadTask.cpp	87.5% <0%> (-4.17%)	⬇️
src/mac/pxWindowNative.h	28.57% <0%> (-2.86%)	⬇️
examples/pxScene2d/src/pxText.h	91.42% <0%> (-2.12%)	⬇️
src/rtScript.cpp	85.71% <0%> (-1.47%)	⬇️
src/rtObject.h	88.7% <0%> (-1.3%)	⬇️
src/mac/pxWindowNative.mm	36.57% <0%> (-1.28%)	⬇️
examples/pxScene2d/src/pxImage.cpp	83.5% <0%> (-1.21%)	⬇️
examples/pxScene2d/src/pxImageA.cpp	88.35% <0%> (-1.18%)	⬇️
... and 86 more

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[dwrobel]

If someone wants to use it on Docker it depends on: #1477

TODO: This PR only enables compilation using v8, later on testRunner_v7.js might also be enabled once #1479 gets resolved.