Support building to a separate directory

[eschaton]

It’d be useful for porting and continuous integration for the project to support building to a separate directory than the source tree and for this directory to be specifiable on the make command line. (For example, make OBJDIR=/builds/fcode-utils/fcode-utils-1.0.3~5 to use the specified directory for build intermediates and products.)

Ideally:

• The build won’t fail if the source directory is read-only and a separate build directory is specified.
• The result of git status after building to a separate build directory will be clean, without needing a .gitignore file.
• Simultaneous builds from the same source directory to separate build directories will all succeed without interfering with each other in any way.