build: don't indent assignments in Makefile

Indented assignment in a Makefile can be interpreted as a command in e.g. GNU Make 3.81 which results in the following error:

make: CPPLINT_QUIET: No such file or directory

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines