Create language_std helper function

A helper function to configure the build's language standard and to
automatically enable or disable C++ support. This allows moving some
more code out of `main`, but it is also preparing for running more than
one build, for splitting the `secp256k1` library.

build.rs

@@ -49,8 +49,9 @@ fn main() -> Result<()> {
     let target = env::var("TARGET").expect("TARGET was not set");
     let mut base_config = cc::Build::new();
 
+    language_std(&mut base_config, "c++17");
+
     base_config
-        .cpp(true)
         .include("depend/zcash/src")
         .include("depend/zcash/src/rust/include/")
         .include("depend/zcash/src/secp256k1/include")
@@ -99,13 +100,6 @@ fn main() -> Result<()> {
         }
     }
 
-    let tool = base_config.get_compiler();
-    if tool.is_like_msvc() {
-        base_config.flag("/std:c++17");
-    } else if tool.is_like_clang() || tool.is_like_gnu() {
-        base_config.flag("-std=c++17");
-    }
-
     if target.contains("windows") {
         base_config.define("WIN32", "1");
     }
@@ -161,3 +155,21 @@ fn is_64bit_compilation() -> bool {
         false
     }
 }
+
+/// Configure the language standard used in the build.
+///
+/// Configures the appropriate flag based on the compiler that's used.
+///
+/// This will also enable or disable the `cpp` flag if the standard is for C++. The code determines
+/// this based on whether `std` starts with `c++` or not.
+fn language_std(build: &mut cc::Build, std: &str) {
+    build.cpp(std.starts_with("c++"));
+
+    let flag = if build.get_compiler().is_like_msvc() {
+        "/std:"
+    } else {
+        "-std="
+    };
+
+    build.flag(&[flag, std].concat());
+}