Bundling with libwebp requires libwebp-dev to be installed

[blaind]

Currently, if bundling with libwebp, libwebp-dev needs to be (unnecessarily?) installed. I think the cause is here:

https://github.com/qnighy/libwebp-sys2-rs/blob/master/build.rs#L235, smoke test building, which requires header files (https://github.com/qnighy/libwebp-sys2-rs/blob/master/src/smoke.c#L1)

Might be related to #4

Not sure, but would e.g. executing this suffice?

echo "int main() { return 0; }" | gcc -lwebp -x c -

status code ($ echo $?) will be zero if libwebp is found, otherwise non-zero

Edit: although the smoke test now checks the libwebp functioning more thoroughly...