Merge branch 'use_correct_variable' into 'master'

Replace $have_png with $use_png

See merge request !88
7 jobs for master in 25 minutes and 21 seconds (queued for 1 second)
Status Job ID Name Coverage
  Prep
passed #6736765
fedora image

00:00:17

 
  Test
passed #6736766
fedora autotools build

00:25:04

passed #6736767
fedora meson build

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passed #6736770
meson android arm64 fedora

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passed #6736771
gst-macos-10.15
meson macOS

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passed #6736768
docker windows 1809
meson vs2017 amd64

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passed #6736769
docker windows 1809
meson vs2017 x86

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