Move /src to /subprojects

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

subprojects/libutil/environment-variables.hh

@@ -0,0 +1,67 @@
+#pragma once
+/**
+ * @file
+ *
+ * Utilities for working with the current process's environment
+ * variables.
+ */
+
+#include <optional>
+
+#include "types.hh"
+#include "file-path.hh"
+
+namespace nix {
+
+/**
+ * @return an environment variable.
+ */
+std::optional<std::string> getEnv(const std::string & key);
+
+/**
+ * Like `getEnv`, but using `OsString` to avoid coercions.
+ */
+std::optional<OsString> getEnvOs(const OsString & key);
+
+/**
+ * @return a non empty environment variable. Returns nullopt if the env
+ * variable is set to ""
+ */
+std::optional<std::string> getEnvNonEmpty(const std::string & key);
+
+/**
+ * Get the entire environment.
+ */
+std::map<std::string, std::string> getEnv();
+
+#ifdef _WIN32
+/**
+ * Implementation of missing POSIX function.
+ */
+int unsetenv(const char * name);
+#endif
+
+/**
+ * Like POSIX `setenv`, but always overrides.
+ *
+ * We don't need the non-overriding version, and this is easier to
+ * reimplement on Windows.
+ */
+int setEnv(const char * name, const char * value);
+
+/**
+ * Like `setEnv`, but using `OsString` to avoid coercions.
+ */
+int setEnvOs(const OsString & name, const OsString & value);
+
+/**
+ * Clear the environment.
+ */
+void clearEnv();
+
+/**
+ * Replace the entire environment with the given one.
+ */
+void replaceEnv(const std::map<std::string, std::string> & newEnv);
+
+}