Uses XDG_CACHE_HOME if available (#8585)

* compiler/options: use XDG_CACHE_HOME if set

* doc/nimc: update documentation

compiler/options.nim

@@ -482,7 +482,7 @@ include packagehandling
 
 proc getOsCacheDir(): string =
   when defined(posix):
-    result = getHomeDir() / ".cache"
+    result = string getEnv("XDG_CACHE_HOME", getHomeDir() / ".cache")
   else:
     result = getHomeDir() / genSubDir
 

doc/nimc.rst

@@ -159,7 +159,8 @@ Generated C code directory
 The generated files that Nim produces all go into a subdirectory called
 ``nimcache``. Its full path is
 
-- ``~/.cache/$projectname(_r|_d)`` on Posix
+- ``$XDG_CACHE_HOME/$projectname(_r|_d)`` or ``~/.cache/$projectname(_r|_d)``
+  on Posix
 - ``$HOME/nimcache/$projectname(_r|_d)`` on Windows.
 
 The ``_r`` suffix is used for release builds, ``_d`` is for debug builds.