How can I pass a macro definition from “make” command line arguments (-D) to C source code?

Call the make command this way:

make CFLAGS=-Dvar=42

And be sure to use $(CFLAGS) in your compile command in the Makefile. As @jørgensen mentioned, putting the variable assignment after the make command will override the CFLAGS value already defined in the Makefile.

Alternatively, you could set -Dvar=42 in another variable than CFLAGS and then reuse this variable in CFLAGS to avoid completely overriding CFLAGS.