Perform a fresh configuration of the build tree

* src/gpg4win.mk.in ($(stampdir)/stamp-$(1)-02-configure,
$(stampdir)/stamp-$(1)-ex-02-configure,
$(stampdir)/stamp-$(1)-native-02-configure): Add --fresh to cmake.

This removes any existing CMakeCache.txt files (and related files) left
over from a previous build to avoid problems with outdated cached
values.