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"