buildsys: fix detection of silent make
The MAKEFLAGS variable contains all the flags without the leading dash.
From the GNU make manual:
This variable is set up automatically by make to contain the flag
letters that make received. Thus, if you do ‘make -ks’ then MAKEFLAGS
gets the value ‘ks’.
Signed-off-by: Mike Frysinger <vapier@gentoo.org>