Makefile.flags: restrict Wno-constant-logical-operand and Wno-string-plus-int options for clang

these options were added in b4ef2e3467d8e980ccf13c9dd342459c013b455f commit
gcc shows unrecognized command-line option warnings

Signed-off-by: Biswapriyo Nath <nathbappai@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
1 file changed