Revert "by default use the system memory allocator"

It seems that even on Linux the system allocator is worse than our
internal allocator. e.g:
http://perf.libreoffice.org/perf_html/chitest_of_cppu_sc_on_vm139.details.html

This reverts commit 006d30b1a29e2c471a96c0f4658beea8cb86b338.
diff --git a/sal/rtl/alloc_global.cxx b/sal/rtl/alloc_global.cxx
index 6961063..60dbc9c 100644
--- a/sal/rtl/alloc_global.cxx
+++ b/sal/rtl/alloc_global.cxx
@@ -35,11 +35,7 @@ AllocMode alloc_mode = AMode_UNSET;
static void determine_alloc_mode()
{
    assert(alloc_mode == AMode_UNSET);
    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;
    alloc_mode = (getenv("G_SLICE") == NULL ? AMode_CUSTOM : AMode_SYSTEM);
}

/* ================================================================= *