BUILD: makefile: warn about unknown USE_* variables

BUILD: makefile: warn about unknown USE_* variables

William suggested that it would be nice to warn about unknown USE_*
variables to more easily catch misspelled ones. The valid ones are
present in use_opts, so by appending "=%" to each of them, we can
build a series of patterns to exclude from MAKEOVERRIDES and emit
a warning for the ones that stand out.

Example:

  $ make TARGET=linux-glibc  USE_QUIC_COMPAT_OPENSSL=1
  Makefile:338: Warning: ignoring unknown build option: USE_QUIC_COMPAT_OPENSSL=1
    CC      src/slz.o