gnucash has historically supported storing passwords for database
backends with libsecret when HAVE_LIBSECRET is defined. The code is
still present, but support for detecting libsecret's availablity was not
ported over when the build system was converted to cmake. This change
restores the missing detection.
This includes removal of the now unused make-gnucash-potfiles.in,
checking for CMakeLists.txt rather than Makefile.am in gnc-vcs-info,
upating the HACKING file,
and generally updating references to autotools.
I have kept "Makefile.*" exclude patterns in our CMakeLists.txt files
because they may still be lingering in the source directory from
previous autogen.sh runs. At some point these should probably be
removed as well still, together with the gitignore references to them.
