meson.build: Require a recent version of libpng

meson.build: Require a recent version of libpng

According to https://gitlab.com/qemu-project/qemu/-/issues/1080#note_998088246
QEMU does not compile with older versions of libpng, so we should check
for a good version in meson.build. According to repology.org, our supported
host target operating systems ship these versions:

             Fedora 35: 1.6.37
     CentOS 8 (RHEL-8): 1.6.34
             Debian 11: 1.6.37
    OpenSUSE Leap 15.3: 1.6.34
      Ubuntu LTS 20.04: 1.6.37
         FreeBSD Ports: 1.6.37
         NetBSD pkgsrc: 1.6.37
         OpenBSD Ports: 1.6.37
              Homebrew: 1.6.37
           MSYS2 mingw: 1.6.37

So it seem reasonable to require at least libpng version 1.6.34 for
our builds.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1080
Message-Id: <20220623174941.531196-1-thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>