Test C and C++ compilation explicitly

tests/BUILD.bazel

@@ -82,6 +82,11 @@ cc_binary(
     srcs = ["cc-test.cc"],
 )
 
+cc_binary(
+    name = "c-test",
+    srcs = ["c-test.c"],
+)
+
 # Test that nixpkgs_cc_configure is selected.
 cc_toolchain_test(
     name = "cc-toolchain",

tests/c-test.c

@@ -0,0 +1,7 @@
+#include "stdio.h"
+int main() {
+        puts("Hello world\n");
+        // Ensure that this is compiled as C, template is a keyword in C++.
+        int template = 0;
+        return template;
+}

tests/cc-test.cc

@@ -1 +1,5 @@
-int main() { return 0; }
+#include <iostream>
+int main() {
+        std::cout << "Hello world\n";
+        return 0;
+}