Make sure linking works even when CC is set to C++ compiler

This can happen when using a module such as `ExtUtils::CppGuess` which
sets `CC` and `CCFLAGS` to C++ compiler specific values. When combined
with `Module::Build` which calls `ExtUtils::CBuilder::Base::have_compiler`
with those values, the branch that checks for a C compiler is run with
those C++ compiler specific values which causes a linker error.

dist/ExtUtils-CBuilder/lib/ExtUtils/CBuilder/Base.pm

@@ -205,7 +205,13 @@ sub have_compiler {
     print $FH q<namespace Bogus { extern "C" int boot_compilet() { return 1; } };> . "\n";
   }
   else {
-    print $FH "int boot_compilet(void) { return 1; }\n";
+    # Use extern "C" if "cc" was set to a C++ compiler.
+    print $FH <<EOF;
+#ifdef __cplusplus
+extern "C"
+#endif
+int boot_compilet(void) { return 1; }
+EOF
   }
   close $FH;