Fix failing tests on Linux

GCC does not accept an extern "C" storage class specifier along with
definition of the global variable. Split declaration and definition to
overcome this.

tests/ffi/tests.cc

@@ -13,6 +13,9 @@
 extern "C" void cxx_test_suite_set_correct() noexcept;
 extern "C" tests::R *cxx_test_suite_get_box() noexcept;
 extern "C" bool cxx_test_suite_r_is_correct(const tests::R *) noexcept;
+extern "C" bool PmrDeleterForC_callback_used;
+
+bool PmrDeleterForC_callback_used{false};
 
 namespace tests {
 
@@ -139,8 +142,6 @@ std::unique_ptr<C> c_return_unique_ptr() {
   return std::unique_ptr<C>(new C{2020});
 }
 
-extern "C" bool PmrDeleterForC_callback_used{false};
-
 void PmrDeleterForC::operator()(C* obj) {
     alloc.destroy(obj);
     alloc.deallocate(obj, 1);