tests fail: "test-lock" and "test-thread_create" with gcc 4.6/4.8

[Assaf Gordon]

Hello,

On Ubuntu 14.04 LTS with GCC 4.6.4 and 4.8.2, the tests "test-lock" and 
"test-thread_create" fail.
They do not fail on same system with GCC 4.4.7 or Clang-3.3 .

Confirmed also by Daiki Uenu:
 http://lists.gnu.org/archive/html/bug-libunistring/2014-09/msg00005.html

From a brief look it seems the "pthread_cancel" is using weak linking, and is 
somehow not properly found by the binary:
 http://lists.gnu.org/archive/html/bug-libunistring/2014-09/msg00002.html

It was found through libunistring but I've also reproduced it with the latest 
gnulib directly (v0.1-210-g4738e5a), with:
    ./gnulib/gnulib-tool --create-testdir --dir=test-thread --with-tests thread
    cd test-thread
    ./configure && make && make check


After running "./configure CC=xxx" with different compilers, the only difference I could spot (besides the 
obvious string change of "gcc-4.4" vs "gcc-4.8") is the following two additional lines appearing in 
"config.log" with gcc 4.6/4.8:

    COLLECT_GCC=gcc-4.8
    COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.8/lto-wrapper

Using gcc-4.4 and clang-3.3 do not produce these lines in "config.log".
"config.h" is identical in all versions.

I can investigate further if you point me where to look.

- Assaf