gnucash master: Set C and C++ standards the modern Cmake way, and set C++ to C++17.

[John Ralls]

Updated	 via  https://github.com/Gnucash/gnucash/commit/931cf50f (commit)
	from  https://github.com/Gnucash/gnucash/commit/1d4d244c (commit)



commit 931cf50f69a37d038009210ff2eed701989be85e
Author: John Ralls <jralls at ceridwen.us>
Date:   Sat Oct 12 16:39:09 2019 -0700

    Set C and C++ standards the modern Cmake way, and set C++ to C++17.
    
    Take 2, accepts pre-1.67 versions of boost that have been patched to
    remove boost::locale's dependence on auto_ptr.



Summary of changes:
 CMakeLists.txt | 35 +++++++++++++++++++++++++++--------
 1 file changed, 27 insertions(+), 8 deletions(-)