Recipes to run the preprocessor

[jscrane]

See here.

For detecting which libraries to include in the build, and for generating function prototypes, (just) the preprocessor is run. For this, the recipe.preproc.macros recipe exists. This recipe must run the preprocessor on a given source file, writing the preprocessed output to a given output file, and generate (only) preprocessor errors on standard output. This preprocessor run should happen with the same defines and other preprocessor-influencing-options as for normally compiling the source files.

This came to light trying to compile src/TommyPROM/HardwareVerify: it fails due to a missing function prototype.