C++ 11 and Boost

[John Ralls]

I’m getting quite close to merging the current C++-build branch into master again, and this brings two significant changes:
* A C++-11 capable compiler is required; the test in configure looks for the compiler accepting either -std=c++11 or -std=c++0x.
* Boost version 1.50 or greater will be required. There’s nothing magic about that version, it was just the oldest of the ones on the various VMs I’ve got. If someone would rather have an older minimum version, I can change it.

Note that the compiler in the old MinGW-11 based build in packaging/win32 doesn’t support C++11, so anyone who’s still using that should switch to Geert’s new gnucash-on-windows scripts; clone it from code.gnucash.org if you have privs there or github.com/Gnucash if you don’t.

Regards,
John Ralls