Workaround a rounding difference in the strtof function in Centos 6. (#290)

* Workaround a rounding difference in the strtof function in Centos 6.

This difference causes the spec test const.wast to fail, so disable the
test on platforms where we detect the rounding difference occurs.

Cargo.toml

@@ -38,6 +38,10 @@ errno = "0.2.4"
 rayon = "1.1"
 wasm-webidl-bindings = "0.4"
 
+# build.rs tests whether to enable a workaround for the libc strtof function.
+[target.'cfg(target_os = "linux")'.build-dependencies]
+libc = "0.2.60"
+
 [workspace]
 members = [
   "misc/wasmtime-rust",