Use the SRC_DIR variable when finding sources
OBJS is now generated from the list of source files
Generate version.c for accessing the Git version
Make BUILD_DIR only if it doesn't exist

This also fixes the problem that everything is build always. Now, only
the changed source files are compiled.
%.s also depends on the BUILD_DIR existing
Generate the list of assembler source files as well with $(wildcard)
Merge pull request #16 from eras/master

Makefile enhancements; embed Git version