add cfg the way num-bigint does it

Author: benmkw

build.rs

@@ -4,5 +4,14 @@ fn main() {
         println!("cargo:rustc-cfg=has_int_exp_fmt");
     }
 
+    let std = if ac.probe_sysroot_crate("std") {
+        "std"
+    } else {
+        "core"
+    };
+    if ac.probe_path(&format!("{}::convert::TryFrom", std)) {
+        autocfg::emit("has_try_from");
+    }
+
     autocfg::rerun_path(file!());
 }

src/from.rs

@@ -4,6 +4,7 @@ use std::convert::TryFrom;
 macro_rules! impl_try_from {
     ( $($name:ty),* => $into:ty ; $approx:ident) => {
         $(
+        #[cfg(has_try_from)]
         impl TryFrom<$name> for Ratio<$into> {
                 type Error = ();
                 paste::paste! {
@@ -16,13 +17,15 @@ macro_rules! impl_try_from {
             }
         )*
 
+        #[cfg(has_try_from)]
         impl TryFrom<f32> for Ratio<$into> {
             type Error = ();
             fn try_from(n: f32) -> Result<Self, ()> {
                 $approx(n, 10e-20, 30).ok_or(())
             }
         }
 
+        #[cfg(has_try_from)]
         impl TryFrom<f64> for Ratio<$into> {
             type Error = ();
             fn try_from(n: f64) -> Result<Self, ()> {