エラーログは以下

Exception: Your system's gstreamer libraries are out of date (we need at least 1.12). If you're unable to install them, let us know by filing a bug!

  File "/Users/ykonomi/servo/python/servo/build_commands.py", line 286, in build
    env = self.build_env(target=target, is_build=True)
  File "/Users/ykonomi/servo/python/servo/command_base.py", line 612, in build_env
    if self.needs_gstreamer_env(target):
  File "/Users/ykonomi/servo/python/servo/command_base.py", line 560, in needs_gstreamer_env
    install them, let us know by filing a bug!")

原因の箇所. check_gstreamer_lib では, subprocess.call(["pkg-config", "--atleast-version=1.14", "gstreamer-1.0"], stdout=PIPE, stderr=PIPE) == 0
が実行される。どうやら、pkg-configの実行が原因でエラーになっている模様。

    def needs_gstreamer_env(self, target):
        try:
            if check_gstreamer_lib():
                return False
        except:
            # Some systems don't have pkg-config; we can't probe in this case
            # and must hope for the best
            return False
        effective_target = target or host_triple()
        if "x86_64" not in effective_target or "android" in effective_target:
            # We don't build gstreamer for non-x86_64 / android yet
            return False
        if sys.platform == "linux2":
            if path.isdir(self.get_gstreamer_path()):
                return True
            else:
                raise Exception("Your system's gstreamer libraries are out of date \
(we need at least 1.12). Please run ./mach bootstrap-gstreamer")
        else:
                raise Exception("Your system's gstreamer libraries are out of date \
(we need at least 1.12). If you're unable to \
install them, let us know by filing a bug!")
        return False

pkg-config gstreamer-1.0 --debug を実行したところ libffi がインストール
されていないと出力されたため、改めて入れ直した。すると、以下のログが出力された。どうやら、/usr/local へのシンボリックリンクが生成されていないため、エラーになっていた模様。

==> Reinstalling libffi
==> Downloading https://homebrew.bintray.com/bottles/libffi-3.2.1.high_sierra.bottle.tar.gz
######################################################################## 100.0%
==> Pouring libffi-3.2.1.high_sierra.bottle.tar.gz
==> Caveats
libffi is keg-only, which means it was not symlinked into /usr/local,
because some formulae require a newer version of libffi.

For compilers to find libffi you may need to set:
  export LDFLAGS="-L/usr/local/opt/libffi/lib"

For pkg-config to find libffi you may need to set:
  export PKG_CONFIG_PATH="/usr/local/opt/libffi/lib/pkgconfig"

==> Summary
🍺  /usr/local/Cellar/libffi/3.2.1: 16 files, 297.0KB

素直に、export PKG_CONFIG_PATH="/usr/local/opt/libffi/lib/pkgconfig"
を実行して解決した。