make env! optional in src/lib.rs

Users of plugins may not want to use a build.rs file to generate the
plugin, as the plugin will not need to know the location of protoc.
Removing the need for a build.rs file simplifies the integration with
build systems other than cargo, such as bazel, through the use of cargo
raze and rules_rust.

prost-build/src/lib.rs

@@ -808,12 +808,16 @@ where
 
 /// Returns the path to the `protoc` binary.
 pub fn protoc() -> &'static Path {
-    Path::new(env!("PROTOC"))
+    // Leave these as optional, for users who do not want to use build.rs, which is not needed by
+    // the protoc plugin users.
+    Path::new(option_env!("PROTOC").unwrap())
 }
 
 /// Returns the path to the Protobuf include directory.
 pub fn protoc_include() -> &'static Path {
-    Path::new(env!("PROTOC_INCLUDE"))
+    // Leave these as optional, for users who do not want to use build.rs, which is not needed by
+    // the protoc plugin users.
+    Path::new(option_env!("PROTOC_INCLUDE").unwrap())
 }
 
 #[cfg(test)]