Build.sh: work around Git for Windows' lack of stat(1)

The Git for Windows installer does not install the stat(1) utility. But
it has GNU date(1). And that version of date(1) has the -r option
which we can use instead of stat(1) to get at the mtime of a file.

Noticed by Olexandra Ovsiy in bug 628.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>