Revert to lower versions of mingwrt and w32api as per mingw recommendations

Attention: you should remove the installed versions of both packages
           before rerunning the installer script the next time !
           you can do so with these commands in your msys shell:
           mingw-get.exe remove mingwrt
           mingw-get.exe remove w32api
Use full path to perl right after installation

At this point perl is normally not in the PATH yet.
Avoid > in echo statements

This is interpreted as redirect to file.
Update strawberry perl to latest version
Change automated build frequency for maint branch

From now on check for changes nightly and rebuild if there was a change.
This is just like the master branch. The weekly restriction was a left over
from the subversion era, where each change in trunk would als trigger a build
in the stable branch. This is no longer the case in git.