mk_app: Create MacOS X application bundle for FriCAS.

Prerequisites:
 - Xcode (tested with 4.6.2)
 - Command Line Tools for Xcode
 - MacOS 10.7.5

Notes:
 - To build FriCAS for MacOS 10.6.8 (Snow Leopard) with newer Xcode versions,
   you need to download Xcode 4.3.3, find and copy 'MacOSX10.6.sdk' directory to
   /Application/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.6.sdk

TODO:
 - Put application bundle into an Apple disk image (.dmg)
 - Enable computations with GMP library.

Run 'mk_app build' script to obtain FriCAS.app directory with (hopefully)
working application bundle.  Note that the script doesn't use FriCAS sources
from the repository, but rather downloads full version from sourceforge.net.

If you wish to build FriCAS with another version of SBCL Common Lisp compiler,
please change SBCLVER variable.

Run 'mk_app dist' to create compressed archive with FriCAS bundle.

To clean up build directory issue 'mk_app clean' command.
