Merge src/makefile_lib and src/makefile_sharedlib into src/makefile.

This required to pass CXXFLAGS on the make command-line, and therefore
I took this opportunity to also pass CGAL_MAKEFILE on the command-line,
which in turn allowed to get rid of the prepare_lib_makefiles function.

Scripts/developer_scripts/create_internal_release

@@ -455,7 +455,6 @@ sub CreateSrcTestDir()
     print "Creating $VERSION/test/src directory ...\n";
     chdir "$RELEASEDIR/$VERSION" or die;
     system('cp', '-r', 'src', 'test');
-    rename('test/src/makefile_lib','test/src/makefile');
     open(CGAL_TEST,">test/src/cgal_test");
     print CGAL_TEST <<'EOF';
 #! /bin/sh