libmicrohttpd: fix thread detection on ARC and Blackfin

libmicrohttpd: fix thread detection on ARC and Blackfin

Due to compiler deficiences, gcc on ARC and Blackfin doesn't define
_REENTRANT, which the latest version of the ax_pthread.m4
autoconf-archive macros looks at to determine whether thread support is
available or not.

Fixing the compilers is definitely the right thing to do, but it's not
doable for pre-built external toolchains like the Blackfin one, so for
now, we simply define _REENTRANT manually on ARC and Blackfin. If more
packages are affected by this problem, a more general solution will have
to be found.

Fixes
http://autobuild.buildroot.net/results/593/593b5050473a83a9ddcada9de39f8f14ab38d554/
http://autobuild.buildroot.net/results/ff0/ff058a378b91740b5b399f32a6b375fbc3c8df06/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>