meson: don't set gnu99 as default c_std

Part-of: <gstreamer/orc!55>
7 jobs for no_cstd_gnu99 in 6 minutes and 43 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #5498582
android fedora arm64

00:06:43

passed #5498576
debian sid

00:00:55

passed #5498581
gst-ios-13.2
ios arm64

00:00:36

passed #5498580
gst-macos-10.15
macos x86_64

00:00:40

manual #5498579
gstreamer-windows 1809 windows docker allowed to fail manual
msys2
passed #5498577
gstreamer-windows 1809 windows docker
vs2017 amd64

00:00:48

passed #5498578
gstreamer-windows 1809 windows docker
vs2017 x86

00:00:48