in ltmain do not execute empty commands, disable libssp for arm x-builds
  this hopefully doesn't have any side-effects, and fixes the following error:
    /bin/sh ../../libtool --tag CXX --tag disable-shared   --mode=link arm-linux-musleabihf-c++     -Wl,-O1 -Wl,-z,relro -Wl,--gc-sections  -std=gnu++11 -prefer-pic -D_GLIBCXX_SHARED -fno-implicit-templates -Wall -Wextra -Wwrite-strings -Wcast-qual -Wabi  -fdiagnostics-show-location=once   -ffunction-sections -fdata-sections  -frandom-seed=libc++11convenience.la  -o libc++11convenience.la  chrono.lo condition_variable.lo debug.lo functexcept.lo functional.lo future.lo hash_c++0x.lo hashtable_c++0x.lo limits.lo mutex.lo placeholders.lo regex.lo shared_ptr.lo system_error.lo thread.lo fstream-inst.lo string-inst.lo wstring-inst.lo   libtool: link:
   make[5]: *** [libc++11convenience.la] Error 1