Win32 build: rename patch files to consistently end in .patch

This way they will be checked in/out in proper line ending style on all
supported platforms.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@22714 57a11ea4-9604-0410-9ed3-97b8803252fd
Win32 build: some more patchfile cleanups

Wrap patching code in conditional in most cases to only run patch if
there actually is a patch file available.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@22715 57a11ea4-9604-0410-9ed3-97b8803252fd