macosx: Make build more portable and general

This broadens the OS X build support to include old and current OS
X/Xcode, and Homebrew/MacPorts.

Support pkgbuild/productbuild + metadata from Mosh-1.2.4-3 package
Search for protobufs in /{usr,opt}/local
Make build select pkgbuild if available, PackageMaker if not
Make PackageBuilder build work on OS X 10.5, XCode 3.1, Macports
Do version substitution
Use generic names for c/c++/cpp
Make build script arch-indepdendent
Changes install dir from /usr to /usr/local

Closes #633.

Signed-off-by: John Hood <cgull@glup.org>