Remove unused partial book handling.
Compile LibQOF as C++

This is a minimal change to get LibQOF and only LibQOF to compile as C++.
There are no functional changes, just casts to satisfy the rather pickier
C++ compiler and wrapping declarations with extern "C" { } to maintain C
mangling so that the libgncmod_qof will work with the rest of the program.

A note about renaming gnc-numeric.c to gnc-numeric.cpp: Dunno if it's a
clang bug, but it wouldn't link test-math with the -xc++ flag; renaming
the one file got around the problem. We'll likely have to do the same for
other test programs and for gnucash-bin.c.
Merge branch 'c++-build'

First merge, libqof only, and no functional changes yet.