Use "VPATH" and ".PATH" for compiling subdirectory source files in Make

GNU Make (gmake on BSDs) and BSD Make (bmake on Linux) handle source files in subdirectories differently:

• GNU Make places compiled object files next to the source files, in their subdirectory
• BSD Make places compiled object files in the Makefile's directory

This inconsistency is a problem for my current project that uses a polyglot Makefile that works on with both Makes.

I'm trying to explore this space as little as possible, but my current solution is to use VPATH for GNU Make and .PATH for BSD Make -- these both support searching for source files in subdirectories (while placing object files in the root--i.e. the same directory as the Makefile). This is handy for e.g. when you're pulling in code via Git submodules, but you're compiling a single binary.

Example:

# Support both GNU Make and BSD Make

VPATH = .:lua:lpeg:md4c/src:chtml
.PATH = ./ lua/ lpeg/ md4c/src/ chtml/