by default use the system memory allocator

Anything except an unset G_SLICE or the value always-malloc results in
the custom allocator. We might revert that change if it causes issues
but currentlz it looks like our custom allocator performs quite badly in
some situations.

Change-Id: Ib867f7ba0de4d599aa045c28aaab9b644d619beb

sal/rtl/alloc_global.cxx

@@ -35,7 +35,11 @@ AllocMode alloc_mode = AMode_UNSET;
 static void determine_alloc_mode()
 {
     assert(alloc_mode == AMode_UNSET);
-    alloc_mode = (getenv("G_SLICE") == NULL ? AMode_CUSTOM : AMode_SYSTEM);
+    const char* alloc_string = getenv("G_SLICE");
+    if (!alloc_string)
+        alloc_mode = AMode_SYSTEM;
+    else
+        alloc_mode = strcmp(alloc_string, "always-malloc") == 0 ? AMode_SYSTEM : AMode_CUSTOM;
 }
 
 /* ================================================================= *