pciutils: Fix build with external toolchains

pciutils: Fix build with external toolchains

Passing GNU_TARGET_NAME implies that we have a 4-group triplet (e.g:
arm-openwrt-unknown-linux), and this works fine with pciutils'
lib/configure operating system detection. If we have an external
toolchain (e.g: mipsel-linux-gnu), the configure script is all confused
and ends-up picking up i386-ports, which is not valid on the target
architecture.

Finally, we are not passing TARGET_CPPFLAGS as we should, so let's do
that correctly.

Signed-off-by: Florian Fainelli <redacted>