Move /src to /subprojects

This will facilitate breaking up Nix into multiple packages for each
component with Meson.

subprojects/libexpr/eval-gc.hh

@@ -0,0 +1,53 @@
+#pragma once
+///@file
+
+#include <cstddef>
+
+#if HAVE_BOEHMGC
+
+#  define GC_INCLUDE_NEW
+
+#  include <gc/gc.h>
+#  include <gc/gc_cpp.h>
+#  include <gc/gc_allocator.h>
+
+#else
+
+#  include <memory>
+
+/* Some dummy aliases for Boehm GC definitions to reduce the number of
+   #ifdefs. */
+
+template<typename T>
+using traceable_allocator = std::allocator<T>;
+
+template<typename T>
+using gc_allocator = std::allocator<T>;
+
+#  define GC_MALLOC_ATOMIC std::malloc
+
+struct gc
+{};
+
+#endif
+
+namespace nix {
+
+/**
+ * Initialise the Boehm GC, if applicable.
+ */
+void initGC();
+
+/**
+ * Make sure `initGC` has already been called.
+ */
+void assertGCInitialized();
+
+#ifdef HAVE_BOEHMGC
+/**
+ * The number of GC cycles since initGC().
+ */
+size_t getGCCycles();
+#endif
+
+} // namespace nix