Fix check for flex
Fails in do_compile at times see below
cp dist/src/nsswitch.yy.c obj/src/nsswitch.yy.c
cp: cannot create regular file 'obj/src/nsswitch.yy.c': No such file or directory
Makefile:78: recipe for target 'obj/src/nsswitch.yy.c' failed
make: *** [obj/src/nsswitch.yy.c] Error 1
Signed-off-by: Khem Raj <raj.khem@gmail.com>
Cc: Armin Kuster <akuster808@gmail.com>