meson: don't set gnu99 as default c_std

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7 jobs for master in 7 minutes and 1 second (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #5498848
android fedora arm64

00:07:01

passed #5498842
debian sid

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passed #5498847
gst-ios-13.2
ios arm64

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passed #5498846
gst-macos-10.15
macos x86_64

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manual #5498845
docker windows 1809 gstreamer-windows allowed to fail manual
msys2
passed #5498843
docker windows 1809 gstreamer-windows
vs2017 amd64

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passed #5498844
docker windows 1809 gstreamer-windows
vs2017 x86

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