linux: Restore datapath and userDataPath calculations

In a95e810, removed datapath and userDataPath calculations by
mistake. This commit restores them. In addition, check on macOS and
Linux that the environment HOME, because at least artificially you
can create an environment where it doesn't.

Fixes: #487 ("Linux Paths")

src/common/SurgeStorage.cpp

@@ -136,6 +136,13 @@ SurgeStorage::SurgeStorage()
 
    memset(&audio_in[0][0], 0, 2 * BLOCK_SIZE_OS * sizeof(float));
 
+#if MAC || LINUX
+   const char* homePath = getenv("HOME");
+   if (!homePath)
+      throw Surge::Error("The environment variable HOME does not exist",
+                         "Surge failed to initialize");
+#endif
+
 #if MAC
    char path[1024];
    FSRef foundRef;
@@ -161,8 +168,16 @@ SurgeStorage::SurgeStorage()
    }
 
    // ~/Documents/Surge in full name
-   sprintf( path, "%s/Documents/Surge", getenv( "HOME" ) );
+   sprintf(path, "%s/Documents/Surge", homePath);
    userDataPath = path;
+#elif LINUX
+   const char* xdgDataPath = getenv("XDG_DATA_HOME");
+   if (xdgDataPath)
+      datapath = std::string(xdgDataPath) + "/Surge/";
+   else
+      datapath = std::string(homePath) + "/.local/share/Surge/";
+
+   userDataPath = std::string(homePath) + "/Documents/Surge";
 #elif WINDOWS
    PWSTR localAppData;
    if (!SHGetKnownFolderPath(FOLDERID_LocalAppData, 0, nullptr, &localAppData))