contrib/buildsystems: redirect errors of the dry run into a log file

contrib/buildsystems: redirect errors of the dry run into a log file

Rather than swallowing the errors, it is better to have them in a file.

To make it obvious what this is about, use the file name
'msvc-build-makedryerrors.txt'.

Further, if the output is empty, simply delete that file. As we target
Git for Windows' SDK (which, unlike its predecessor msysGit, offers Perl
versions newer than 5.8), we can use the quite readable syntax `if -f -z
$ErrsFile` (available in Perl >=5.10).

Note that the file will contain the new values of the GIT_VERSION and
GITGUI_VERSION if they were generated by the make file. They are omitted
if the release is tagged and indentically defined in their respective
GIT_VERSION_GEN file DEF_VER variables.

Signed-off-by: Philip Oakley <philipoakley@iee.org>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>