plotutils: fix erroneous use of the -flat_namespace flag

None of our usual patches apply to their `configure` script, but,
conveniently, we don't need one if we set `MACOSX_DEPLOYMENT_TARGET`
appropriately.

Let's also pass `--without-x` on macOS to avoid opportunistic linkage
with X11 libraries.

This is needed for bottling on Monterey. See Homebrew#94212.

Formula/plotutils.rb

@@ -30,13 +30,18 @@ def install
  # Fix usage of libpng to be 1.5 compatible
  inreplace "libplot/z_write.c", "png_ptr->jmpbuf", "png_jmpbuf (png_ptr)"
 
+ # Avoid `-flat_namespace` flag.
+ ENV["MACOSX_DEPLOYMENT_TARGET"] = MacOS.version.to_s
+
  args = %W[
  --disable-debug
  --disable-dependency-tracking
  --disable-silent-rules
  --prefix=#{prefix}
  --enable-libplotter
  ]
+ # Prevent opportunistic linkage to X11
+ args << "--without-x" if OS.mac?
 
  system "./configure", *args
  system "make"