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",

build.rs

@@ -154,5 +154,30 @@ fn ignore(testsuite: &str, name: &str) -> bool {
             (_, _) => false,
         };
     }
+
+    #[cfg(target_os = "linux")]
+    {
+        // Test whether the libc correctly parses the following constant; if so,
+        // we can run the "const" test. If not, the "const" test will fail, since
+        // we use wabt to parse the tests and wabt uses strtof.
+        extern "C" {
+            pub fn strtof(s: *const libc::c_char, endp: *mut *mut libc::c_char) -> libc::c_float;
+        }
+        if unsafe {
+            strtof(
+                b"8.8817847263968443574e-16" as *const u8 as *const libc::c_char,
+                std::ptr::null_mut(),
+            )
+        }
+        .to_bits()
+            != 0x26800001
+        {
+            return match (testsuite, name) {
+                ("spec_testsuite", "const") => true,
+                (_, _) => false,
+            };
+        }
+    }
+
     false
 }