If we already defined `CFLAGS=-D_FORTIFY_SOURCE=1`, it will still check `-D_FORTIFY_SOURCE=2` and
add it to CPPFLAGS. In this case, we redefine `_FORTIFY_SOURCE` to both 1 and 2 if compiler accepts `-D_FORTIFY_SOURCE=2`.
Fix this issue by checking whether `_FORTIFY_SOURCE` was defined or not.
If defined, do nothing. If not, then check `-D_FORTIFY_SOURCE=2`.
BTW, OpenWrt's buildroot uses `-D_FORTIFY_SOURCE=1` for most targets by default.
Syrone Wong
Max Lv