Fix PR 90142: contrib/download_prerequisites uses test ==

Since == is not portable, it is better to use = in contrib/
download_prerequisites.  The only place == was used is inside
the function md5_check which is used only on Mac OS X.

Tested on Mac OS X as:
./contrib/download_prerequisites --md5
Both with all files having the correct checksum and one with a broken one.

contrib/ChangeLog:

	* download_prerequisites (md5_check): Replace == inside
	test with = to be more portable.