Move /src to /subprojects

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

subprojects/libutil/environment-variables.cc

@@ -0,0 +1,51 @@
+#include "util.hh"
+#include "environment-variables.hh"
+
+extern char ** environ __attribute__((weak));
+
+namespace nix {
+
+std::optional<std::string> getEnv(const std::string & key)
+{
+    char * value = getenv(key.c_str());
+    if (!value)
+        return {};
+    return std::string(value);
+}
+
+std::optional<std::string> getEnvNonEmpty(const std::string & key)
+{
+    auto value = getEnv(key);
+    if (value == "")
+        return {};
+    return value;
+}
+
+std::map<std::string, std::string> getEnv()
+{
+    std::map<std::string, std::string> env;
+    for (size_t i = 0; environ[i]; ++i) {
+        auto s = environ[i];
+        auto eq = strchr(s, '=');
+        if (!eq)
+            // invalid env, just keep going
+            continue;
+        env.emplace(std::string(s, eq), std::string(eq + 1));
+    }
+    return env;
+}
+
+void clearEnv()
+{
+    for (auto & name : getEnv())
+        unsetenv(name.first.c_str());
+}
+
+void replaceEnv(const std::map<std::string, std::string> & newEnv)
+{
+    clearEnv();
+    for (auto & newEnvVar : newEnv)
+        setEnv(newEnvVar.first.c_str(), newEnvVar.second.c_str());
+}
+
+}