disable lto test for clang for now

it works on debian but not on the outdated travis ubuntu

.travis.yml

@@ -165,11 +165,7 @@ matrix:
 
     # Link time optimization
     - env: SANITIZER=1 COMPILE_LTO=1 BUILDOPTIONS='--with-cc=gcc-5 --with-m64 --with-travis-valgrind'
-    - env: SANITIZER=1 COMPILE_LTO=1 BUILDOPTIONS='--with-cc=clang-7 --with-m64 --with-travis-valgrind'
-      addons:
-        apt:
-          packages:
-            - llvm-7
+    #- env: SANITIZER=1 COMPILE_LTO=1 BUILDOPTIONS='--with-cc=clang-7 --with-m64 --with-travis-valgrind'
 
     # GCC for the x86-64 architecture with restricted limb sizes
     # formerly started with the option "--with-low-mp" to testme.sh