Move /src to /subprojects

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

subprojects/libutil/windows/environment-variables.cc

@@ -0,0 +1,45 @@
+#include "environment-variables.hh"
+
+#include "processenv.h"
+
+namespace nix {
+
+std::optional<OsString> getEnvOs(const OsString & key)
+{
+    // Determine the required buffer size for the environment variable value
+    DWORD bufferSize = GetEnvironmentVariableW(key.c_str(), nullptr, 0);
+    if (bufferSize == 0) {
+        return std::nullopt;
+    }
+
+    // Allocate a buffer to hold the environment variable value
+    std::wstring value{L'\0', bufferSize};
+
+    // Retrieve the environment variable value
+    DWORD resultSize = GetEnvironmentVariableW(key.c_str(), &value[0], bufferSize);
+    if (resultSize == 0) {
+        return std::nullopt;
+    }
+
+    // Resize the string to remove the extra null characters
+    value.resize(resultSize);
+
+    return value;
+}
+
+int unsetenv(const char * name)
+{
+    return -SetEnvironmentVariableA(name, nullptr);
+}
+
+int setEnv(const char * name, const char * value)
+{
+    return -SetEnvironmentVariableA(name, value);
+}
+
+int setEnvOs(const OsString & name, const OsString & value)
+{
+    return -SetEnvironmentVariableW(name.c_str(), value.c_str());
+}
+
+}