Meta/Make: simplify CFLAGS

Author: gitster

Make

@@ -272,7 +272,7 @@ ${make-make} \
      ${DEVELOPER:+"DEVELOPER=YesPlease"} \
      SPATCH_FLAGS="--all-includes --recursive-includes --patch ." \
      SPATCH_BATCH_SIZE=16 \
-     CFLAGS="-g -Wall -Wunused -Wdeclaration-after-statement -std=gnu99${O:+ $O}" \
+     CFLAGS="-g -Wdeclaration-after-statement ${O:+ $O}" \
      ${tests:+"T=$tests"} ${tests:+UNIT_TESTS=} \
      "$@"
 status=$?