Some type mismatch fixes to make it build with clang in MacOSX Mavericks.
These may not be the best fixes, but they make things build again with
XCode 6.1.1 in MacOSX 10.9.5.
Tell Guile (and others) to look in install/lib/gnucash for ligraries.
Without this Guile can't find libgncmod-engine and GnuCash won't start.
I don't know how this worked before, perhaps because the correct path
was in src/bin/overrides/gnucash-env.in
Update XCode project for recent C++ related changes