Improve handling of 'srcdir' environment variable

Throw a more meaningful exception if the srcdir variable is not set, rather
than a construction error from inside std::string.

radiant/test/VFSFixture.h

@@ -12,16 +12,24 @@ struct VFSFixture
     vfs::VirtualFileSystem::ExtensionSet pakExtensions;
     vfs::SearchPaths searchPaths;
 
+    // Get the srcdir environment variable (set by Automake)
+    std::string srcdir() const
+    {
+        const char* envVal = getenv("srcdir");
+        if (envVal)
+            return std::string(envVal);
+        else
+            throw std::runtime_error("srcdir not set");
+    }
+
     VFSFixture()
     {
         // Setup the output stream
         GlobalOutputStream().setStream(std::cout);
 
         // Configure search paths and extensions
         pakExtensions.insert("pk4");
-        searchPaths.insertIfNotExists(
-            std::string(getenv("srcdir")) + "/test/data/vfs_root"
-        );
+        searchPaths.insertIfNotExists(srcdir() + "/test/data/vfs_root");
 
         // Initialise the VFS
         fs.initialise(searchPaths, pakExtensions);