[Buildroot] [PATCH] valgrind: valgrind/valgrind.h missing if --disable-tls enabled

[Stefan Froberg]

Enabling valgrind support and then doing debug build of Firefox
started configure script nagging about missing valgrind/valgrind.h file.

Found out that if --disable-tls is in VALGRIND_CONF_OPT then
the valgrind installation process will not include the previously mentioned file.

So this patch will enable/disable TLS for valgrind depending of
what you have in your compiler BR2_GCC_ENABLE_TLS setting.