Enable LTO again
    
    It had reportedly been disabled due to failing CppunitTest_sw_apitests in
    LO 6.4.  My recent upstream check with --enable-lto on master towards LO 7.1
    found CppunitTest_sw_apitests to work fine, and only found one other failing
    test in CppunitTest,xmlsecurity_signing, but which is new for LO 7.1 and not yet
    present in LO 7.0 (see 
    800eebfa82106c509310ed43bef38a7a4ad4451f%5E!> "Database document apparently
    needs to be closed before it is disposed" and 
    +/2f2246d22e2a8ccbc1dc3e6f5243734a61edf270%5E!> "external/cppunit: Run tests in
    deterministic order".
    
    From a successful scratch build of this commit, it looks like
    CppunitTest_sw_apitests already works fine in LO 7.0, and that there are no
    other issue with LTO in LO 7.0.