🎨 Provide better control over freestanding

Problem:
- The `SIMULATE_FREESTANDING` macro applies to multiple repos. Sometimes we
  might want finer-grained control over which elements of which repos are
  freestanding-compatible, because toolchains differ in what they support.

Solution:
- Introduce `CONC_FREESTANDING` for this repo control.
- Defining `SIMULATE_FREESTANDING` will also control `CONC_FREESTANDING`.

Author: elbeno

include/conc/concurrency.hpp

@@ -1,13 +1,14 @@
 #pragma once
 
-#ifdef SIMULATE_FREESTANDING
+#include <conc/concepts.hpp>
+#include <conc/detail/freestanding.hpp>
+
+#ifdef CONC_FREESTANDING
 #define HAS_MUTEX 0
 #else
 #define HAS_MUTEX __has_include(<mutex>)
 #endif
 
-#include <conc/concepts.hpp>
-
 #if HAS_MUTEX
 #include <mutex>
 #endif

include/conc/detail/freestanding.hpp

@@ -0,0 +1,5 @@
+#pragma once
+
+#ifdef SIMULATE_FREESTANDING
+#define CONC_FREESTANDING
+#endif

test/fail_no_conc_policy.cpp

@@ -1,4 +1,4 @@
-#define SIMULATE_FREESTANDING
+#define CONC_FREESTANDING
 
 #include <conc/concurrency.hpp>
 

test/freestanding_conc_injected_policy.cpp

@@ -1,4 +1,4 @@
-#define SIMULATE_FREESTANDING
+#define CONC_FREESTANDING
 
 #include <conc/concepts.hpp>
 #include <conc/concurrency.hpp>