[build] Enable PYTHONOPTIMIZE for linux

If I build for Pi using unified buuld from Ubuntu, then python doesn't work
/opt/xbmc-bcm/xbmc-dbg/arm-linux-gnueabihf/arm-linux-gnueabihf/lib/python2.6

contains the .pyo files, but not the .py files.

Enabling PYTHONOPTIMIZE is required for the .pyo files to be used.
Do this by default on linux platforms. (It also gives a small performance benefit)

xbmc/interfaces/python/XBPython.cpp

@@ -564,7 +564,8 @@ bool XBPython::OnScriptInitialized(ILanguageInvoker *invoker)
 
 
     // Darwin packs .pyo files, we need PYTHONOPTIMIZE on in order to load them.
-#if defined(TARGET_DARWIN)
+    // linux built with unified builds only packages the pyo files so need it
+#if defined(TARGET_DARWIN) || defined(TARGET_LINUX)
     setenv("PYTHONOPTIMIZE", "1", 1);
 #endif
     // Info about interesting python envvars available