gnucash maint: Add support for libsecret to cmake

[John Ralls]

Updated	 via  https://github.com/Gnucash/gnucash/commit/8cae602e (commit)
	from  https://github.com/Gnucash/gnucash/commit/6c03d07e (commit)



commit 8cae602e66f593bb2af1c5bbcbd3e48476a58596
Author: Benjamin Gordon <ben at bxg.org>
Date:   Tue Jun 12 23:19:48 2018 -0600

    Add support for libsecret to cmake
    
    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.



Summary of changes:
 CMakeLists.txt                     | 5 +++++
 cmake/README_CMAKE.txt             | 5 ++---
 gnucash/gnome-utils/CMakeLists.txt | 3 ++-
 3 files changed, 9 insertions(+), 4 deletions(-)