Correct uom feature hygiene in macro generated code.

Add macros to condtionally include contents based on features `uom` was
compiled with rather then features in the destination crate. Resolves
#98 and #100.

src/features.rs

@@ -0,0 +1,83 @@
+#[macro_export]
+#[cfg(feature = "autoconvert")]
+macro_rules! autoconvert {
+    ($($tt:tt)*) => { $($tt)* };
+}
+
+#[macro_export]
+#[cfg(not(feature = "autoconvert"))]
+macro_rules! autoconvert {
+    ($($tt:tt)*) => {};
+}
+
+#[macro_export]
+#[cfg(any(feature = "autoconvert", test))]
+macro_rules! autoconvert_test {
+    ($($tt:tt)*) => { $($tt)* };
+}
+
+#[macro_export]
+#[cfg(not(any(feature = "autoconvert", test)))]
+macro_rules! autoconvert_test {
+    ($($tt:tt)*) => {};
+}
+
+#[macro_export]
+#[cfg(feature = "autoconvert")]
+macro_rules! not_autoconvert {
+    ($($tt:tt)*) => {};
+}
+
+#[macro_export]
+#[cfg(not(feature = "autoconvert"))]
+macro_rules! not_autoconvert {
+    ($($tt:tt)*) => { $($tt)* };
+}
+
+#[macro_export]
+#[cfg(feature = "serde")]
+macro_rules! serde {
+    ($($tt:tt)*) => { $($tt)* };
+}
+
+#[macro_export]
+#[cfg(not(feature = "serde"))]
+macro_rules! serde {
+    ($($tt:tt)*) => {};
+}
+
+#[macro_export]
+#[cfg(feature = "si")]
+macro_rules! si {
+    ($($tt:tt)*) => { $($tt)* };
+}
+
+#[macro_export]
+#[cfg(not(feature = "si"))]
+macro_rules! si {
+    ($($tt:tt)*) => {};
+}
+
+#[macro_export]
+#[cfg(feature = "std")]
+macro_rules! std {
+    ($($tt:tt)*) => { $($tt)* };
+}
+
+#[macro_export]
+#[cfg(not(feature = "std"))]
+macro_rules! std {
+    ($($tt:tt)*) => {};
+}
+
+#[macro_export]
+#[cfg(test)]
+macro_rules! test {
+    ($($tt:tt)*) => { $($tt)* };
+}
+
+#[macro_export]
+#[cfg(not(test))]
+macro_rules! test {
+    ($($tt:tt)*) => {};
+}

src/lib.rs

@@ -247,6 +247,9 @@ pub mod num {
     }
 }
 
+#[macro_use]
+mod features;
+
 #[macro_use]
 mod storage_types;